Skin of flayed better than Forti or duriels?
you need life leech to keep a merc alive
the 3 options are
chains of honor
skin of the flayed one + protector's stone
greyform + protector's stone
Protector's stone alone gives pretty comparable DPS boost to the %ed/str on chains of honor, and also gives more % mf, and has the 1% chance to cast fade which only comes into play in long TZ games
mercs are already at 62% all resists by default and 75% poison with cure, so you get 62/62/62/75 with flayed one 75/75/62/75 with greyform, 75% all with coh.
cold resist doesn't matter at all (only significant source is frost nova on death), 62% lightning is enough to survive souls. Fire resist matters the most, though warlock builds with tainted also get resist fire aura for merc for free
So you can have 7% leech or 5% leech and +13% fire/cold resist, or you can use CoH for 75% all and 8% leech and +20% ed might or +2 prayer, but not have the spiffy bonuses of the jewel