I finally did it! i killed 100% of the enemies in the game. This means Bob, too. If you have found him you may find this interesting. Along with my mage i used Mezey, Kela and Joska.

The problem with this setup is my mage dealt almost no damage once the water filled up, but he dealt a TON of damage before this. The key to my strategy is that i equipped Joska with Pestilence and War weapons, which have that bonus plague effect whenever he deals poison damage. He also has plague damage passives unlocked. Also i gave him the Poison damage potion, since you are allowed to use potions during the 2nd half of the battle. So save your poison potions for the underwater part. While you are attacking Bob while underwater, your Plague poison damage will still build up and damage him over the turns which was the key to beating him for me. Before the water filled up i did get him down below 5,000 though. maybe more like down to 4,000. before the water fills up, make sure to use as many of his poison specials as possible to get tons of plague counters on Bob. Also in general, equip as many abilities as you can that deal damage over turns (DOT). joska has at least two of them, and Kela has Bleeding that is very good here also. Mage's fire damage over turns is also very very good.

Also i used mezey's offensive buff ability Don Your Armor a lot. My mage has the fire and lightning damage passive talents unlocked, with archmage spellbook 3 equipped. Also i would first use his fire elemental then unload fire on him, and i used the Sun party ability also, making sure to use my mage to activate it because of the fire damage bonuses and archmage spellbook. also i would use blizzard and the lightning one whenever the fire ones were cooling down. the party ability that does ice damage over turns is good too but make sure you do that early, since once the water fills up, Bob will shed any DOT effects you had on him.

Finally i suited up Kela in the full set of King Attila's gear, to get through 100% of Bob's armor. Plus equipped that horn that gives 10% physical damage bonus and 10% health and i was using her bleeding, stalagmite, Bash, Summon Underworld Creature, Grueling, and that other one that hits all the enemies. I'm not positive that its necessary to use attila's gear, but i am 99% sure Bob is a "boss" (instant kills don't work on him from what i can tell- i tried those and it always said "immune"). Also make sure you equip your strongest character with herbal tea. i didnt really need energy potion. So i recommend giving one character the herbal tea, and giving joska the poison potion, and giving kela the Troll Blood potion that does physical damage, and then maybe give the 4th character either the water of life or another potion that does damage, like if your a mage give your mage the fire potion to take advantage of fire bonuses. thru kela's passive ability that sometimes causes fear, and thru some of the other gear that i had equipped, i found that every so often Bob was getting stunned and feared and stuff like that and i didnt really worry about defense abilities like i would in a normal fight. i didnt even use mezey's taunt once. Hope all this helps and let me know if you found a good strategy to take down Bob!

An alternative to Mezey is Kampos - if you equip him with strong melee weapons and/or Attila's sword, then have him do the Rage bear attack he can do a TON of damage to bob, especially if he is already in Bear form. since there is only one enemy to hit, his Rage ability will hit 2-4 times. i've seen him do 200+ damage wiht one rage attack to a single enemy before! i think that instead of mezey i could have used kampos to maybe make things more efficient, since mezey didnt really have too many relevant special abilities - but i do think Don your armor is very good and also i used his flank and For the King moves as well but mostly just attacked with basic melee.