Help a party of five 20th level PCs defeat an Ancient Red Dragon

[Dragonblade]

If the party is 20th level and the dragon is 30th level, then just have the Star Blood grant a +10 bonus to attacks and all defenses and for the duration of the encounter. It also grants 50 temp HPs that last the entire encounter. Done. Its simple and easy to remember and generally works mathematically.

Next I recommend finding the stat block of a solo ancient dragon that is close to level 30 as possible and using that as a baseline with some tweaks. Shift the level to 30 by adding +1 to attacks and defenses for every level it needs to boost it to 30. If the stat block comes from a pre-MM3 source, then add +15 damage to any attack or aura the creature has and subtract 2 from all its defenses (after you have level shifted it to 30). If your players are highly optimized, add 20 to all its damage instead of +15.

Add about 10 HP per level you need to increase it to reach level 30, otherwise leave HP alone. Have the dragon make saves at the beginning of the dragon's turn, instead of at the end, AND if it fails a save, or has a condition that can't be saved against, it can choose to automatically shake off any condition or ongoing effect that afflicts it, regardless of current duration, but in exchange it takes 15 damage per each condition or ongoing effect it ends in this way.

That will solve the issue of player's stun locking it. Your dragon gets to still take its actions, but the PCs conditions aren't simply ignored or handwaved away. That's how I would do it.

 

[Dragonblade]

If your party has two leaders and are ruthlessly efficient tactical killing machines like my players are, I may even go so far as to give the dragon two initiative counts and let it act twice a round. But that could be too much. It depends on what you think your players can handle. I like to push my players to the edge of a TPK in big boss fights, and make them dig deep into their bag of tricks. They have surprised me with crazy combos every time. One thing I love about 4e.

 

[Dragonblade]

In the interest of full disclosure, I'll mention that I use the Escalation Die from 13th Age in my 4e games.

 

[pemerton]

On action denial - I like [MENTION=2804]Dragonblade[/MENTION]'s ideas. I also agree about pushing the players hard in this sort of fight, and making them dig deep.

Some ideas of my own - which can be combined with Dragonblade's in whole or part, or used instead, or ridiculed mercilessly, as appropriate:

* I'll re-emphasise the suggestion of giving your dragon two initiative counts - this helps with action denial, and with action economy more generally. For extra viciousness, let it act on a 3rd initiative account when bloodied! (MV dragons, and Calastryx, show some ways of approaching this.) Another good feature of this approach (from my point of view, at least) is that it rewards players who think hard about the initiative sequence and make clever use of delaying to coordinate the party's actions so as to work well around the dragon's initiative count.

* Give it free actions (so it can do stuff when dazed) or no action abilities - say some sort of breath or fire or aura ability (so it can do stuff when stunned). The MV beholder and purple worm have some examples along these lines.

* Another idea is to let is spend an action point as no action to shed a condition, gain a save etc. Combine this with an ability to extinguish/swallow minions to add APs, so it can stock up!

* Make sure it has a viable close or area attack so even if blinded it can do something without a -5 to hit.

* Decide how you want to handle immobilisation/slowing - which can be as bad as dazing! Calastryx has Rip and Tear, which lets her throw a melee target multiple squares away, thus escaping fighter lockdown. You might want to think about something like that (maybe a wing buffet?). Also think about a ranged attack if appropriate - especially if the dragon can retreat behind lava and attack from a distance (can your defenders fly?).​

On minions/cohorts - there's nothing wrong with some devils to warm things up, but I would strongly recommend having some cohorts present with the dragon also. Flying and/or ranged would be good, because you can then try to shift pressure onto other PCs besides the defenders (who presumably will try and lock down the dragon). In the post I linked to upthread, the encounter used salamander archers for this purpose. For flying devils, stat up some 18th-20th level spine devil minions - in an encounter on the weekend I used (among other things) 6 21st level Vrock minions and these worked well as an irritant to keep the PCs (and players) on their toes. There are also some other devils that I've used recently in levelled-up, MM3-ed form: Assassin devil (a nice lurker - if a controller or ranged striker takes up a nice position on a ledge above the lava, drop one of these on them!), and the Indwelling Devil, which has a dominate ability - dominate is a nice way to add a bit of a twist to an encounter without just denying a player actions!

 

[Quickleaf]

Once they get to the ancient red, she will be resting on a platform which, if I go with the alternate ending, is the Great Seal that imprisons the adversary. The Great Seal exists within a great hellish cavern complete with magma flows, falls and lakes, and perhaps some sort of minor demon minions - abishai? - flying around and attacking them. I like the idea of the dragon employing spells, perhaps psychic attacks, and then blasting them with fire. If and when they get to her, she may be easier to dispatch. She will be tough, but she is decrepit and not quick anymore.

This sounds very much like a non-standard red dragon...but in your OP you ask for a standard ancient red dragon updated to MM3 guidelines. So are you looking for a custom designed monster or just an updated ancient red dragon?

You should check out my Dragon Mountain thread for a sample 3-stage red dragon "Infyrana" designed as a controller (it's in post #104 in the 4th pdf file).

http://www.enworld.org/forum/showthread.php?331204-Dragon-Mountain-(4e-conversion)/page11

She has spellcasting (mostly pyromancy) and a few psychic effects. Right now she's statted up as a level 15 solo (i.e. adult) but I'm considering upping her to near 22nd (i.e. elder) because my party already defeated a level 15 red dragon when they were level 11. That would make her terrifying and rather unbalanced to a party of 13th/14th level PCs, but I'm also considering including some "tricks" to the fight and breaking the monster design rules.