On Volk Island, there's a barge (the floating structure that's closest to you as you look at the final boss' house from the closest Supply Point). Kill all the enemies that are hanging around on the barge. As you walk from the front to the back of the barge, there's a door to your left under the crossbar. Enemies will spawn inside the room and emerege to fight you. As you pass the door (you have to walk all the way past it), the first wave will come. Kill them. Stand there and face the door. When the next wave of enemies emerges inside the room, your reticle will go red and the red dots will pop up on the HUD. When the door opens, shoot a hothead round or lob a grenade in there and they all die. After a couple waves, the bodies pile up and prop the door open, letting you throw grenades in as soon as you see them spawn inside the room. This will continue for about 10 or so waves of enemies, and then they'll stop. Head back to the supply point and reload, then go back and do it all over again and again. I was able to max out my explosives and firearms abilities using this method in a relatively short amount of time. Had I had the patience, I probably could have done the same with the strength ability. This also can get you the mad bomber achievment.