The Damned Alchemist walks a little more slowly than most ground-based enemies, and jumps a little more quickly than it normally walks. Some instances possess an innate protection field that makes all enemies in its area of influence immune to all damage, similar to Ambrosia. This effect is not shared by all Damned Alchemists, but it is a fairly common trait. The effect of the field does not extend to itself, but can affect other nearby Damned Alchemists.
The Damned Alchemist's only attack is melee, so it is generally safe to dispatch from range. The variants which possess a protection field are very high-priority targets, as it is completely impossible to damage other enemies in the vicinity while the Damned Alchemist remains near them. Alternatively, a player can kite the other enemies away from their protector until they are vulnerable, as Damned Alchemists move much slower than most enemies.
If two protection field-bearing Damned Alchemists happen to be near each other, your only options are to flee or attempt to kite one of them far away enough for it to lose the other's protection. If there are other enemies nearby in this situation, it can be safest to kite them away first and dispatch them before attempting to deal with the Damned Alchemists themselves.