Hmmm. I wrote down some notes for Namtar because I was considering a "Vicious travel guide". Maybe I should solicit more information? I suppose I have a playstyle that focuses more on hitting things hard than spells, so maybe that's why I never had trouble with the fourth form.
Form 1: 476 HP, 112 attack, 50% magic resist, poison, mana burn, magic attack. The debuffs make regen fighting impossible; the combo of mana burn and magic resists makes magic little use in general. Also, I'm usually no better than level 8 here. Typically this is where I use my whupaz, quicksilver, and reflex potions. If I can somehow take a hit at level 7, a level 7->8 level-up during this one is helpful. High magic resists make this phase easier.
Form 2: same stats, Curse Bearer, Corrosive, Weakening Strike. Taking a couple hits of the debuffs is not so bad. Magic works better here, but save your potions for Form 3. The curses make resistances difficult.
Form 3: 238 HP, 75 attack, 75% physical resist, fast regen. (Forms 3-6 also have Undead and Bloodless.) This takes 7 fireballs. Hope you saved mana and/or schadenfreude potions, or have a level-up available. I suppose the RBS would work too, and a decent melee attack with first strike could save you maybe one fireball. (Then you lose 10 HP and 2 mana)
Form 4: 318 HP, 75 attack, Cowardly, Retaliate:Fireball, Berserk at 50%. You'll want to chase him into a corner so that you control the regeneration. This is like Form 1 without the debuffs, fewer HP, only gets the big attack once he's half dead, and you're probably a higher level. (Then you lose 10 HP, 2 mana, 5 damage, and all resistances.)
Form 5: 238 HP, 37 attack, Berserk at 75%, 3x death protections, cowardly, magic attack. The attack is weak enough that there's plenty of room to regen fight. Unless you converted the Fireball glyph, you can use either that or just plain attacking to burn off the death protections. (Then you lose 10 HP, 2 mana, 5 damage, and take 10x weakening and corrosion, not that it matters)
Form 6: 79 HP, 1 damage, Cowardly, Retaliate:Fireball (?!). Even with the weakening and corrosion you just took, this is clearly not supposed to be an actual threat.