By Island 3, your squad should be well-upgraded. Focus on how your mechs' abilities complement each other. Can your Combat Mech push a Vek into the Artillery Mech's line of fire? Can your Prime Mech clear a path for another mech to reach a critical target? Consider equipping secondary weapons that fill gaps in your squad's capabilities, such as a smoke-generating weapon for crowd control or a repair ability for sustained fights. Pilot skills become even more impactful here.
