round 60? Damn! Then you definitely do not need advice from me lol
I guess since the Prison power ups dont respawn thats also not much use to you!
I was using the lift thing by the docks as an escape route, so activate the sniper trap (which I think you can upgrade), then hold out as long as possible around the docks, get the lift back upto the prison, unfortunately this is where is becomes difficult,
Either hold out here and get the lift back down, or try and sprint through the Prison to a more open area. Its a shame the roof doesnt go anywhere else, would be a good looping run
As you may have guessed, that is indeed the fastest strategy. At least the fastest that I've found. Just make sure you get them all with the trap each time and that's the fastest it will go.
I usually run them out of the cafeteria, make a tiny loop on the left side to gather them tight, then loop back in and hit the trap. That way, I get most if not all of them and am right back in the cafeteria.