When starting a Co-op campaign, instead of logging into your main Gamertag as player one, log in as player two. Doing this from the beginning or while replaying chapters will let you play as Dominic Santiago. There are several achievements that you will unlock playing as him. Additionally, if you start as Dominic from the beginning, you will not gain any less points then you would as
Marcus and it will save you a lot of time.
Easy group kills:
Once you get the Torque Bow, wait until you see groups of Locusts, then shoot one in the head. After you wait a few seconds, it should blow up and kill all or most of them.
Two player path block :
Use the following trick while playing in two player mode and you reach a difficult part with a doorway. Have one player die in the doorway. This will create a block that the Locust cannot get past.
They will usually line up behind your dead partner, allowing you a safe zone to easily pick them off. Once it is clear, just revive your partner and move on.
Chainsaw cuts through objects:
The chainsaw on your Lancer has the ability to cut through many things if you get close to it and start it up. This includes doors, walls, and boxes.
Closing Emergence Holes:
When an Emergence Hole opens up, toss a grenade into it. This will automatically close the hole, and therefore keep the locusts from coming out. The Hammer Of Dawn can easily destroy Emergence Holes. You must activate it by targeting an enemy first, and then aim it
toward the Emergence Hole.
Fire a Torque Bow arrow into an Emergence Hole. This is somewhat challenging to do, due to the straight line of fire given by the bow. However, if you are close enough to fire while the line of fire is arced, it is possible to make it into the hole.
A Torque Arrow stuck in the top of the hole will close it as well.