Frozen Elite glitch
From Halopedia, the Halo wiki
The Frozen Elite Glitch can be found on the Truth and Reconciliation Level. Go through the level like normally until you get into the ship. Go through the first and second battle rooms, now you should be going to the Spirit dropship bay. Before leaving to go there kill all of your Marines then go through until you reach two Jackals, kill them, then you will see one Elite and some Grunts. Kill the Grunts but not the Elite then, right before you round the corner past the battle, throw a Plasma Grenade way past the Elite then round the corner and trigger the checkpoint. He should be frozen. you can throw Plasma grenades on him and they won't blow, even hit him with Needler rounds and they will stick to him. However, once you round the corner a second time, he will explode.