Only ran into it once so far, but killed its regular version (ie not powered up by Witch's Maze 5+).

Chances are, you're not completing enough relevant defensive symbols for late game. If you're low on unlocks, then prioritize accessories with symbols that grant Barrier/Healing/Regen/Max HP since you may be missing enough units to fully access those symbols. Now that you've faced it, you'll see it coming on future runs and know what to prep for.

Its like knowing the Ghost boss is coming and not bringing some form of damage scaling or AoE, only to get walled forever by summons. The Dragon is not particularly difficult to kill based on raw durability, but it tests your ability to either outrace it or tank its moveset.

Every single run, tried to aim for 5 green symbol. After that, try to unlock as many symbol as you can. Team synergy is also ver inportant.