The heavy tacticals have always been able to 2-shot you and the Elite ones will 1-shot you if you don't have the best gear. On a crit they 1-shot anyone and anything so the best tactic is to avoid taking damage. Have a wall nearby or an engineer's cover shield you can hide behind between shots. If you _do_ take damage you have to get out of combat and heal yourself or rely on a teammate to heal you or you _will_ die - quickly. Also make sure engineers put their medical and shield generators in strategic positions behind walls or cover shields so they can't be destroyed by the drones.