Master Ma - not a Merc, but his stat boost still invluable for warriors
Deirf Geiss
Stone King, possibly replaced with Hares the Runic if he drops in price a bit

Here we already have possible (at best) +3 ATK, +3 DEF and +1 SPT boost from characters only. I thought of these as a core:

Weapons (mostly quick-activating ones):

3x Teslarms - Decent chainable +2 ATK and helps get rid of Thunderblast
1x Sorrow Harpoon - Good weapon, chainable on all characters
1x Tantadian Spear - Defense buster
1-2x Dagger of Sufferance

Armor:
1-2x Runic Armor - against Priests
1-2x Runic Gauntlet - Defense against Priests and Mages

Actions:
3x Six Feet Under
2x Determination
1x Back to Noz'Dingard
1x Anathematize - Necros, Admiral, Artrezil, IHAS...
1x Equinox