Level 1. Go to your left. There is a zombie. Do not kill him. Jump over him and keep going left. You will see some crates and a small drop. Try not to knock the crates on the zombies below. There will be an explosive canister. The idea is to bait the zombies toward the canister and blow it up. The difficulties are that sometimes the zombies blow up the canister themselves too soon. It took a few tries, but I got it that way.
Got the Boombastic quest: - wait for a big group to show up (like additional enemy ambushes in a room of enemies, or the small crabs/spiders mostly come in groups) - equip the Milkor (grenade launcher) and start shooting crazy!
I realized you don't have to blow up 5 at once, you just have to do it quickly in in a row, the grenade launcher is a great help with the 6 shot ammo clips since the bazooka has to be reloaded after each shot
reton8's plan could work too, but you need a bit of luck (messed it up a few times :P)
Just got all the quests. There were pretty similar to Zombotron 2, and I'm a bit sad that it didn't give me the quests even though I had completed the game beforehand, but still a fun game nonetheless.
This whole Boombastic discussion just shows how different people's experiences can be -- I got Boombastic just by playing "normally" through the game; it was one of the first quests I got, and I didn't intentionally try to complete it.
I had the same problem -- in level 16 you just hang around the escape capsule for a bit. In other words, don't go into the capsule until the boss appears and you beat him.