Played this on Notdoppler when it first came out, and I got a 4 billion combo 0.o
To make a never-ending system, set up some sentinels on the left wall, as well as launchers coming from the left. The shells will get stuck just bouncing and being zapped at the same time. Once you get your entire floor covered with launchers and proxy mines, you can add turrets and buzzsaws in the air.
Also, once you make a never-ending combo, end it by going to the shop and staying there for a few seconds.
 "You are a... The Sequel"){kind=icon}
 "Demons Took My Daughter"){kind=icon}
 "Epic Combo!"){kind=icon}