Key Skill Gems for Mercenary Build:

Primary Skill Gem: Depending on your skill of choice (e.g., Lacerate, Cyclone), link with Path of exile currency support gems that increase physical damage, bleed chance, or attack speed.

Example Lacerate Links:
Lacerate + Brutality + Multistrike + Melee Physical Damage + Impale + Ruthless.
Example Cyclone Links:
Cyclone + Fortify + Infused Channelling + Pulverize + Melee Physical Damage.
Utility Skills:

Leap Slam or Dash: Essential for mobility.
Blood Rage: Adds attack speed and life leech (but costs life over time).
Molten Shell or Steelskin: Defensive utility to POE currency absorb incoming damage.
Enduring Cry: Grants endurance charges and instant life recovery, boosting survivability.