I always had an issue with this type of thinking.
How does the monster(s) know that the Fighter is "too hard" to hit? After a single swing, does the DM just have the monsters avoid the Fighter from then on? Or does the DM wait for a few rounds? Granted, a fighter in plate and shield is obviously well protected, but that's where spells like Disguise Self (for Eldritch Knights) comes into play (as does the Sentinel feat). And how is a fighter in plate and shield any different than a cleric in plate and shield?
There's also the issue of off tanking. Is a monster off on the side going to ignore a fighter in his face and use a disengage or give the fighter an OA? Isn't this good for the party overall?
There's no doubt that a DM should play the monsters according to their intelligence, but how does a monster know the difference between a fighter in plate and a fighter in +3 plate? Do monsters always avoid every PC in heavy armor and/or shields?
I agree with you. The creature doesnt know what he has rolled, as dice of destiny doesnt exist in the game world. This is basically metagaming, sort of.
Granted though that guys in no armor or leather are generally easier to hit than guys in plates, but purposefully leaving your own armor lower just so enemies will think that they can hit you better is just stupid.
The same should occur with most monsters. It's not like they have minutes to analyze the situation (like the DM and the players do at the table), they have mere seconds to react. The concept of "only attacking the squishies" is something that monsters should rarely do, even above average intelligent monsters. They should attack whomever is closest since whomever is closest would appear to be one of the more predominant threats in the heat of the moment, at least as a general rule shy of special circumstances (like the PC further away just cast a Fireball spell).
Our group came up with another one, similar to this. One person lays down Fog Cloud, and then the next casts Spike Growth over the same area. Was rather effective! If the battle had lasted much beyond that, Thunderwave would have been the perfect follow-up.For your druid, cast spike growth, followed by thunderwave. If foes fail their save on the thunderwave , they are pushed back thru the spikes for extra damage.
Speaking of Delayed Blast, I need to reread the spell descriptions carefully to make sure it's possible or even useful, but could Catapult be used to launch Delayed Blast Fireball's bead or an Otiluke's Freezing Sphere?
For your druid, cast spike growth, followed by thunderwave. If foes fail their save on the thunderwave , they are pushed back thru the spikes for extra damage.
Heck, if the target is already next to the spikes, you don't even need a spell slot! Just cast gust on them to push them 5 feet!