Legendary Creature ACs are not that high. Bless is often in effect against such creatures. Your average chance to hit is +4 or 5 stat +4 proficiency at level 10 and +2 on average bless. If you have a magic item that boosts hit bonus, and I do, that also.
My hit roll is +4 proficiency +4 intel +2 staff =+10 with bless +12.
Generally against AC 18 to 19 for Legendary Creatures my level or slightly above.
That means I hit 70 to 75% of the time. So yes, it is unreasonable to assume 50% to hit chance.
It would be unreasonable to assume that the average 10th level character is carrying around a very rare magical item, that level of rarity usually isn't found by characters until they are L11 or higher. I also wasn't assuming you would be benefiting from a concentration based buff given how you have been talking about the difficulty you have been having maintaining concentration in the face of DC 30 concentration checks. Without those two, you would have had the +8 I assumed. With a 75% chance to hit, Fire Bolt will do an average of 8.8 damage at L10 and 13.2 damage at L11, resulting in Fireball still higher average damage by 60% or 6% respectively. So at L11 with a 75% chance to hit and the assumption that the save for Fireball will succeed 100% of the time, the cantrip finally does almost as much average damage as Fireball used suboptimally against a single target. For the general case situation where it isn't a L10 character with a Staff of Power and the assumption of a perpetual Bless, Fireball outdamages cantrips by the larger margin.
Cover does not work against a breath weapon or AoE attack requiring a Con Save. All cold and poison attacks are Con saves. That's why Rogue evasion is kind of dumb in this edition. Somehow you can evade fire and acid breath weapons, but cold and poison gas are too special to evade for reasons only the game designers can fathom.
If you take cover away from the party against a creature with 80 feet of movement, it moves to you, full attacks, and moves past you disallowing your martials from attacking it or allowing them to be sealed off by Legendary Actions that can do stuff like erect walls or create difficult terrain. As a caster you are almost forced to remain in relatively close proximity to the martials, so highly mobile creatures do not utilize their mobility to kill you off pieacemeal. Cover does not help against highly mobile creatures because they circumvent with their mobility, hit you, and move away all very quickly. Your AoO from reaction is meaningless to them unless they are very low on hit points.
Why is rogue evasion dumb because it doesn't work against two particular breath weapons, one of which makes perfect sense (why would you make a Dex save to hold your breath in the middle of a cloud of poison)? Is it dumb that it works against fire, lightning, and acid breath, as well as fireballs, lightning bolts, thunderwave, flame strike, and a whole host of other spells and attacks?
You went after a dragon in it's lair without making sure your party had the capabilities needed fight it effectively, and then you complain that as a result it ripped your party a new one. Unless you were teleported into the dragon's lair against your will, you chose to have the fight. The abilities needed to deal with those effects are available, although the members of your party may not have chosen to select them at character creation or during advancement. I'll also reiterate, a CR13 dragon by itself is pushing a deadly encounter for a L10 party. The situational advantage it gets from being in it's lair pushes it up a category to beyond deadly. Looking at that encounter in RoT (I hadn't gotten there yet, the group I'm running is still early in HotDQ), in my opinion the characteristics of the lair itself would warrant another category bump, which would go from beyond deadly to the dragon deciding whether BBQ sauce or honey mustard goes better on adventurers.
He is not using me as a crutch. He wanted to play a strength-based sword wielder. He's supposed to be punished for that because of concentration, no ability to purchase magic items, and mobility playing an increased roll with every creature able to move their full speed and do a full attack at any point during the movement? Like I said, it's my fun or his fun and both of our deaths if I don't get him into action.
This topic alone could easily be a thread in itself. I feel that players should be allowed to build the character they want, but they should not expect other members of the party to pick up their slack without asking them first, and definitely not at the expense of the fun of other players. The entire purpose of playing a RPG is to have fun, so if a player's choice prevents other players from having fun, then yes they are doing it wrong and should not make that choice out of respect for the other players. If the player is so focused that the only thing they are willing to play is a single specific character build, that is also a sign of issues because the implication there is that they expect everyone else will have to adjust to accommodate them, but they aren't willing to adjust to accommodate anyone else.
Not every character is a good fit for every party or every adventure, some characters are just flat out less effective than others, and some character concepts that were effective in older editions of the game are less so in the new edition. There are differing schools of though about whether the DM should adjust the adventure to fit the players, or whether the adventures should adapt their characters to fit the adventure, but if neither side of the screen adjust then things are going to go poorly for the party unless they luck into a good fit.
Did your group make characters knowing you would be playing in the HotDQ/RoT modules? Did you make characters together after talking about party composition and what capabilities you wanted to make sure were covered, or did everyone just make a character on their own? The strength-based sword wielder should have at least given a passing thought to "I'm playing a game called Dungeons &
Dragons, that means I'll probably end up fighting
dragons at some point, what will I be able to do to be useful when that time comes?" And if you knew what modules you would be in, maybe they should have thought to themselves "the first module is called Horde of the
Dragon Queen, the second module is called Rise of Tiamat (the goddess of evil
dragons), and the campaign is called Tyranny of
Dragons, maybe this is a subtle hint that fighting
dragons will be a significant element of the campaign that I should be prepared for." Sort of like when someone was creating a ranger for the old Against the Giants campaign, they might have gotten the hint that "giant" would be a good choice as a favored enemy. If they still decide to be focused entirely on melee, they might at least consider investing in the Sentinel feat to help keep the enemies in melee with them so they can take advantage of their own strengths.
The player isn't being "punished" because of concentration, they are trying to ignore the fact that there are trade-offs in their decisions and they apparently only want the positive consequences of their choices without having to figure out how to work with the negative consequences. If you chose to play a low Dex and low Con mage with no armor, Mage Armor, or Shield to maximize you Int, Cha, and focus on all evocation spells, would you say you are being punished because fighters are no longer as good at preventing enemies from getting past, or would you accept that being fragile and vulnerable in combat is a consequence of your own decisions and the price you pay for the strengths you chose? The situation appears to me that the other player made poor choices relative to the new version of the rules and the campaign, and the only way they are mitigating those poor choices is by having someone else to fix the problem for them, hence my view that they are using you as a crutch to compensate for the repercussions of their own decisions. This also appears to bother you to some extent, otherwise you wouldn't be saying things like "it's my fun or his fun and both of our deaths if I don't get him into action" emphasizing that if you don't choose to give up your fun, both characters will die as a result of the other player having his fun.
If you don't want to look at it from an OOC/metagame viewpoint, why hasn't the party done something in character before now to address the fact that they appear to be melee heavy and ranged combat deficient? More than likely they encountered their first dragon at L1, and ranged combat ability would have been a huge asset in that fight. They could have sought out hireling archers, or even gone so far as to say "sorry Sir Smashalot, we have too many members in this band that focus on close combat and you are the weakest/most unlikable/most annoying/least versatile, so we're going to need to let you go so that we can bring in someone to complement the rest of the party better. No hard feelings, and we hope you have a long and happy life while we struggle to not become a dragon's lunch."
Read Hoard of the Dragon Queen module series. See how the set up is. Imagine an intelligent DM taking full advantage of terrain and lair actions.
I've looked at that fight. I agree it is brutal, and unless your party is both well prepared and well suited for the encounter it will go very badly. I question whether an intelligent DM should be maximizing the tactical capabilities of a creature with an 8 Intelligence of a type that is expressly described as "lack[ing] the cunning and tactics of most other dragons." Once again I think it is up to either the DM to adjust the adventure to the party or the party to adjust to the adventure depending on your groups feelings on the matter. But I don't know what an adventure throwing a deadly or worse encounter has to do with the topic of whether wizards are fun or whether concentration is overly limiting, unless your view is that the presence of a wizard in the party in such an encounter should let the party defeat what the system establishes as a deadly or worse encounter and that by single-handedly altering the difficulty scale by their presence the wizard's player will have fun. Legendary Resistance was added to prevent what is intended to be set-piece encounters against legendary creatures from becoming nothing but a speedbump due to a single failed save like would happen in earlier editions. Someone at WotC decided (rightly or wrongly) that the wizard pressing an "I win" button in what should be the most dramatic combat encounters didn't make for a very fun experience for everyone else.
The adventure does make some allowances to try to help though, and I don't know if your group was able to take advantage of them.
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For example, did you interact with the NPC that was the main reason why you were there in the first place? Did they give you the items that the adventure says they offer? Did your party use them? One of the items would have gone a long way towards helping you keep concentration, the others in competent hands would have let the party do a decent chunk of damage to the dragon very quickly and maybe even gotten the dragon to burn through some of it's legendary resistance. Did you find the large cache of scrolls that include both some potentially high damage spells and some defensive spells, including one that doesn't require concentration?
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