There's only the one giant bat that I've ever seen. Leastwise, I've never seen more than the one once I've killed him. I've managed to kill him all three times through the game. The first time he surprised me and came right up to start battle with me and I ended up meleeing or using single-opponent spells on him. The other two times I was watching for him and initiated combat with him while he was still at thrown range and in both cases my ranger took him with an insta-kill in the first round.
He's worth a decent XP if you can get him to come in close enough to initiate combat. Just make sure you don't blind him or he'll fly away. Best place to attract him is by standing on a walkway -- he seems to come closer then than if you are too near a building.
Here's a list of the Alchemy levels needed to produce each type of potion:
Lt. Heal + Bless = Guardian Angel (15) (Worthless)
Lt. Heal + Mod. Heal = Hvy Heal (15)
Mod Heal + Mod Stam = Pickmeup (20)
Stink Bomb + Boom Bomb = Acid Bomb (20)
Lt. Heal + Poison Reduction = Cure Poison (30)
Hvy. Heal + Cure Lt. Cond. = Cure Disease (40)
Hvy. Heal + Hvy. Stam = Restoration (50)
Hvy Heal + Cure Disease = Renewal (50)
Flash Pwdr. + Sneeze Pwdr. = Pandemonium Pwdr. (50) (Best Value)
Concussion Pwdr. + Dessication Dust = Skelton Pwdr (75) (Worthless)
Magic Nectar + Renewal = Resurrecton Pwdr (85)
Fire BOmb + Ice Bomb = Canned Elemental (95)
Of course, there are springs that can heal your problem as well if you haven't used them up - two in Arnika (the pool next to Braffit and the statue below the temple next to the vault with the drape hanging over it) and one in Mt. Gigas caves behind the barracks of the enlisted men - it looks like a grotto. There's also the pool in the Lwr. Monastery that MIGHT also cure draining. If worse comes to worst, just let the character drown/die and then resurrect him and the draining is gone.