Properly geared merc (high dps + Life Leech) can kill immune monsters on Hell but the best benefit is extra aura. Sorc, Trap Assa and Javazon should use Infinity to remove light and fire immunities, other casters should pick Insight for infinite mana and melee can choose between Reaper, Pride and Faith. If you do Chaos (Diablo) runs, you should consider Lawbringer as it will knock undeads (knights) out of the way. You must always hire them on the lowest difficulty for the best stats. You can buy socket and learn more facts about merc on product page.