Alien Hominid/YMMV


 * That One Boss - People may have different answers for this, but the mid-level boss of level 3-2 is an absolute pain in the ass.
 * AKA the electric sockets of doom. If you don't attack the core that glows pink (out of the multiple cores), they'll fire out a nigh-unavoidable attack from all their cores. The only way to avoid this is to stand on the very edge of one of them and crouch (so that you can hit the cores), in which case the soldiers coming from above will land on the sockets, crouch, and shoot you.
 * That One Level - Level 1-2 can be especially annoying on co-op mode if you and your partner aren't careful with how many cars you decide to hijack, and 2-5 has a very difficult vertically scrolling area with flamethrowers, barrels rolling down staircases and a satellite shooting lasers at you from orbit.