Arcade Paradise

Not a guide exactly as this is a pretty straightforward grind, but a reminder that it's cumulative and your total kills is on the top right of the post run stat screen. Also once a weapon is purchased in the store it's added to subsequent run item pools as a possible drop. So no need to double buy, just unlock another type when you have the cash to get more drops.

I'd recommend prioritizing other weapon unlocks before the sniper rifle & flamethrower. Neither are bad, but the sniper has a low rate of fire and the flamethrower, while awesome when firing, has a long reload time that can screw you. The other weapons are much more efficient.

If you're going for

put this one off. My daily tasks have plenty of "get 100 kills in Zombat 2" and the like that you'll get plenty of opportunities to chip away at the number.