Continue upstairs and breach the door for a scene. After the scene, more enemies will pour in the church interior. Proceed ahead and kill the enemies that will appear from the door. Turn around to the right and an MMG will start firing at you. Dig in and take out the enemies before moving forth. Grab the shield ahead when you have the chance then order your partner to take cover behind you by pressing the UP dpad.
Move towards the room where the MMG is located while killing enemies along the way. Breach the door open and quickly kill the gunner to the right then concentrate your fire to the brute before the slow motion sequence finishes. Man the MMG and use it to build your Overkill meter.
After the scene, take out the brute with the grenade launcher then stay in cover as you fend off the enemies. There's a sniper rifle in the position where you dropped so you can use that to pick off enemies from a safe distance and cover. Be careful of the riotshield-wielding enemies as they'll flank you from the sides. Continue clearing the area until every last enemy is dead. Proceed to the exit of the church. Breach it open to complete the section.
I can't open the last door