The Destroyers are not a problem for you as long as you have some guns with 36"+ range. You only need to cause 3 unsaved wounds for them to be perma-dead, no WBB. Just max out vs the massive, massive, massive noob errors your opponent is making.
Priority activity:
1. kill the Destroyers from range on turn 1. They don't get WBB if you kill all 3.
2. use your S5+ shooting attacks to put pressure on the C'tan.
3. forget about lashing the C'tan to keep it away. Use the lash to get the Warriors NEARER.
4. an early assault on the Warriors can give you a Phase Out win by turn 2. This will mean that the C'tan can be pretty much ignored, there isn't enough time for it to do anything.
Put all your force against the weakest aspect of your opponent's army and it will be a brutal lesson in list building for him.
