Give Frederick Spellforged Servitor and armour more befitting a demigod

Spellforged Servitor replaces his Invisibility, which isn't terribly
relevant by the time Frederick is encountered. Spellforged enhances
his high-damage spellset in a way that can be countered through
careful positioning.

I also restore his heavier body armour choices so he'll be a bit less
squishy. The justification for the previous downgrade of Frederick's
armour was the lack of realism/player-symmetry from a monster casting
high-level spells in heavy armour. However those issues aren't more
important than giving appropriate defenses to what's supposed to be
one of the hardest uniques in the 3 rune game.