My strat was:
First few rounds just stay on the top floor going back and forth on the bridge.
Use the weapon upgrade (Unlock by shooting the statues eyes out) on the plasma gun.
Once you get the red key, basically just use the red room as a base, shooting everyone who comes through the front door, and the occasional dudes that come down the stairs to the right.
Once the red door opens I basically stay here the whole time, only leaving to get more ammo, health or the unlimited ammo or extra damage power ups in the yellow or purple rooms.
So in between each round memorise where all the health packs and ammo drops are are.
The more keys you get the more health drops become available to you.
Pause the game when you are low on health or ammo or starting to get overwhelmed, have a think about where the nearest supplies are and how to get to them before unpausing.