This is for third tier of the Gun Runner's Challenge for a challenge called "Crackerjack Timing." Considering time bombs, tin grenades, and long-fuse dynamite is probably the last thing you'd use to kill a cazadore, this method makes it trivially easy, and I haven't seen anyone else talk about or use this method.

Just outside of Jacobstown, towards the Remnants Bunker in the hilly area in-between the two. The reason I choose this location is because there are tons of cazadores here and they respawn about every 3 days. You should be able to get it within a couple runs.

Video Below

So first, its best to have a companion tank for you, I use Lily Bowen. (Also I unequip any weapon I have equipped on her, it limits the chance that she kills them and you don't get the challenge count.) Then, use grenades, dynamite, firebombs, or any other throwable or weapon like the Tesla Cannon that does DOT(Damage Over Time). Then once you have thrown or shot your weapon of choice and the DOT cycle will kill the Cazadore, switch to a time bomb, long-fuse dynamite, or tin grenade. I bound grenades to one and the long-fuse dynamite to two.

The reason this happens is Fallout thinks that whatever explosive is in your hand, its the one doing the damage at the time of the kill, therefore you can use any explosive or weapon that's much easier to use and find to in turn speed through this challenge with ease.

Video:https://www.youtube.com/watch?v=WASJbwl7Ks8&ab_channel=HellenKeller

I'm sure very few people will read/watch this, but hopefully I helped someone out with this quick fix. It makes the tier 3 challenges so much easier to get through. Just now achievement hunting this classic.