After playing through the game on veteran pretty easily I decided to do a run on impossible, and tonight after a couple hours testing I finally 3 starred Veznan's tower on impossible with only free content, finishing my 3 star impossible playthrough. Figured I'd post my setup since this is probably the most difficult stage in all of the franchise and answer some questions about the stage or the game in general. Might make a tierlist or even more detailed post if I feel like it later.
Heroes: Connor w/revive passive, Zefira w/resist shred passive
Spells: reinforcements full top tree, edict full bottom tree, musketeers full top tree
Towers: see screenshots 2 and 3
My main tips:
Max gold prospector on wave 1 right side was a massive difference maker in my run. I felt kind of gold starved in terms of upgrading abilities on previous runs in midgame, but after the first 7-8 waves the gold really felt like it was adding up, and around wave 9 to wave 11 I already unlocked most relevant tower skills(notably scholar mage buff) when in previous runs I couldn't. Required quite a bit of micro with zefira in early stages but i think very worth it.
Dwarven culverin was the second main difference maker in my 3 star run. in the vast majority of other stages I stuck to catapult for its ridiculous range making several overlap every chokepoint. On this level, though, the aoe stun ultimate, higher dps ability, and very importantly the 100% magic resist shred bomb for 150 gold was extremely useful against the demons which all have medium+ magic resist. This allows magic towers to actually dps the thicker impossible difficulty demons.
Stage mechanics tips:
Heroes are invulnerable to damage targeting while moving, this is very useful to abuse in general. When Moloch(big orange fella) does his hand slam or chucks magma elemental balls, your heroes will be safe as long as they are moving while the damage goes through or out of the aoe. Veznan's purple magic projectiles however will damage and kill your heroes if you don't move them far enough from the impact spot (or long enough duration?), this is important because his usage of that spell in one of the earlier waves (4-7? i forgot) can assassinate one of your heroes and screw your defense over. However, in my playthrough Connor just stood back up because of his revive passive; he may be the best hero for this last level because of that and his invincibility.
Moloch's hand slam attack is extremely annoying but can be played around with some knowledge. First, the AOE of his hand slam damage is actually about the entire right side of the stage, much larger than his visual hand. If you're teleporting zefira or another hero out of aoe(which was easier for me to think about) and not just using the invulnerability while moving, you must teleport them to somewhere on the left half of the screen to avoid damage entirely. Denas is usually fat enough to just tank it and heal up. As for your barracks that probably die, you can instantly revive golems and soldiers with royal edict to essentially reset and defend all the crap that spawns after his attacks.
Other thoughts:
The main difficulty (at least that I've found) with this last stage is the contrast between high armor dark knights and slayer enemies and themagic resistant mages and demon enemies. In addition, everything is much tankier due to impossible difficulty, so the artillery upgrade tree that gives them 75% armor bypass and higher base range and damage is more valuable than the miniscule stun and attack speed. Dead enemies do not need to be stunned or blocked.
Scholar mage and archer buffs are absolutely crucial in this impossible mode stage to allow the lower range dwarven culverin tower to cover much more area than usual, using the sulfur bomb to let mages help with magic resist enemies instead of getting stuck on them, and increasing the time all towers shoot at enemies by letting them target earlier and for longer.
Sunray mages are basically barracks with the best dps basic attack in the game and aoe true damage stun. they are much more valuable than barracks overall. In all previous stages i either did not use barracks or only had level 1 versions, replacing them with this tower. In this stage though, other towers are needed a lot to deal with the demons.
If you made it down here thanks for reading. Lmk if you want to discuss thought process or have questions. I'll play this stage again and do all the other content too