Game Spoilers
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The Wizard Slayer->Thief does not need a Fighter's HLAs, as its' creation is for one sole purpose. To be a tough nut to crack. Ironically, it is because of the Mods that make him more powerful, due to the custom items.
A level 20->32 WS->T will have the following to beat the crap out of the final ToB Boss.
Wizard Slayer Base: 24% Magic Resistance
Machine of Lum the Mad: +5% = 29%
Nine Hells: +10% = 39%
Purifier +5: +30% = 69%
Amulet of Power: +5% = 74%
Ring of Gaxx: +10% = 84%
Cloak of Balduran: +25% = 109%
Whoopee! Magic Immunity! Now let us check the physical damage resistances... ...
Defender of Easthaven: 20%
Jan Jansen Armour: +25% = 45%
Ascension ToB Bonus: +25% = 70%
That is one combination. There is another.
DoE: 20%
Ascension: 45%
YellowDragonScaleArmour: 45% vs Slashing/Crushing, 95% vs Piercing
Ronarch's Helm: 45% vs Slashing, 95% vs Piercing/Crushing
Regarding the hassles of playing a level 1 Thief when you dual, this is why it is nice to postpone the reward collection until you dual.