I'm missing something, Warmonger can't happen in Magma Mines that I can think of, with the 100% physical resist boss (except with the Half-Dragon, but then they run into issues with the 100% magic resist boss...)
There are several ways to approach this. We had a little "in-forum" competition over this very situation a while back, and even took it one step further: kill the bosses in opposite order, effectively taking down the 100% resist both boss at full strength.
The most obvious approach is to use Mystera, Binlor, or Jehora Jeheyu to lower the resistances of the bosses. Because the second "bodyguard" is reduced to 1 HP when you knock out the first, you only need to deal 1 point of damage to finish him. Any
damage that gets through his resistances will do. Unless it's been fixed, corrosion damage ignores resistances so you can kill the physical resist boss if you equip the martyr wraps. The really big sword also works here, as does BYSSEPS. The piercing wand is the only resist-down effect that doesn't work; since fireballs cannot even target 100% resist enemies, you can't get your foot in the door with this implement. As a result, you'll need to use a deity if you want to get a magical specialist victory here.
The half-dragon actually runs into a tricky problem when nabbing specialist badges. His standard attack has both a magical and physical component (the main attack is magical, but the knockback attack is physical), so you cannot use your knockback effect if you want your specialist badge.
Whrrgarbl works as well - at least for Warmonger. After killing the magic resist boss, you just need to hit the physical resist boss once to give him a stack of burning, then hit something else to make it ignite and kill him.
I think it might even give Specialist too (the code for that badge is odd), but it's been a while since I tried it.