Take a thorn and shoot chief. The tick will affect him regardless of whether the bullet hits his flesh. Using a tether (which also does significant damage) and a smoke bomb would make Chief unable to do jack shit. Then the Guardian could place a spike grenade on chief, which would do rapid and direct damage. A fusion rifle would easily deal large damage to chief, and a cluster bomb rocket launcher wouldn't help his chances. I know chief is strong and fast, but what would he honestly do against someone just as durable who has better gear?