While still a bit tricky, I found the best place for this was right at the end of level 2.1, at the checkpoint where the big airship is. Leave the bomber alive and run from left to right, causing 4 black wolves to spawn. You may want to kill most of the cannons (not all or the level ends). Then try to group the wolves up and let the bomber throw dynamite onto the ground. If you hit it after it lands, it flies fairly straight, so it shouldn't be too hard to hit a group of wolves with it. Just kill yourself and try again if you run out of wolves.