Haha the bats are indeed a pretty tough fight.
When designing the game, I was fondly reminiscing upon the "Hardest Map Ever" custom maps from Starcraft Broodwar. The makers of those maps combined the terrain and units abilities to create "combat puzzles" that required the player to plan out a sequence of abilities and timings to overcome the encounter.
In these "puzzles" you often had multiple siege tanks with a huge positional advantage going up against 3-6 light infantry units, so it wasn't terribly uncommon for units to get focus-fired or one-shotted due to a mistake. The bats encounter is an example of this design in practice: the "Ring of Frost" spell you start with applies knock back & slow to the swarm of bats, which spreads them out and allows you to pick them off one at a time, but you have to time it just right and reposition yourself for that tactic to work.
Reflecting upon your feedback here, perhaps the "Roguelike" aspect of restarting the level doesn't mesh well with that "Combat-Puzzle" aspiration due to the long reset time? I had at one point considered adding frequent dark-souls-bonfire-style checkpoints every few encounters, but I didn't include those because I found myself enjoying the "high stakes" aspect of the long resets.
Thanks for the details!