Ba'al is level 30, so it should be possible to take him out on level 29 (though another level or two would certainly help, if you want to go do some or all of a flying fortress, or something).
I found my creature most helpful with a mage head. Trying to take out a couple level 21 imps in the fjords at level 17, it was much more useful with a level 2 ranger head than a level 6 warrior head, and I switched to a mage head as soon as I found one after that.
Level 1 of the healing spell comes in handy. For a summon you may want to try the demon, rather than the ghost, since the ghost isn't necessarily reliable about healing you.
Did you check with your alchemist, and see if you can make any stronger healing or rejuvenation potions?
I never cared much for reflect type skills, since the point of building your character is to minimize the damage done. A Static Charge aura on a piece of jewelry did come in handy when I first got to Sentinel Island, though, to take out some wyverns that kept out of bow range most of the time (that aura returns a set amount of magical damage, rather than a percentage of damage done to you).
Level 1 of Evade helps for melee combat.
For a warrior, max your weapon expertise, and get Bleed up to level 10.
Have you visited your enchanter's platform, and made sure you have the best enchantments available? I put Life Line (up to level 9) on most of my armour, aside from a couple of instances to help with resistances. With jewelry I used a couple level 10 healing auras, but generally didn't bother upgrading or changing many (next character I think I may try to max out Static Charge, up to level 9). For weapons of course I used up to level 9 damage aura, and up to level 9 magic damage aura if there was a second slot.
The Hide In Shadows skill could let you heal in peace, while your creature does some damage. Even running around jumping side to side should let you avoid the majority of attacks for a while.