Microsoft Minesweeper

Monsters are pretty common in this game. Start a level, search for arrows and then kill them. Machetes, if found, have unlimited uses. I attempted to reach Level 20 four times, and in doing so, accumulated 75 monster kills. So, just keep playing reasonably long games of adventure mode and this will come naturally. To grind this, you want to balance difficulty and opportunity. Therefore, I'd recommend playing up to Level 10 repeatedly. Later levels get more difficult. You'll encounter plenty of monsters during the first ten levels. If you want, you may even stretch it out to level 15, but beyond that, the difficulty rises, slowing you down. Each 10-15 level game should give you good progress towards this achievement. Happy grinding!

Monsters will sometimes be guarding keys. Collect these and use them to make progress towards the following related achievement:



Also, as noted by Excalibur0126, kills with dynamite count. So if you have extra dynamite, you may not need to find additional arrows. Scour the level for dynamite in addition to pickaxes and other tools. That way, if you're running low on arrows, you can use it.

An easy way to grind this achievement is to scout each level for monsters, find the required amount of weapons (or the machete), then click each monster as your character will automatically dig to where you click. One thing to note is blowing up monsters with dynamite actually counts. With this in mind, a good habit is to clear out your "free" blocks in case there's any hiding in them. Once you've defeated all the monsters on the level, or there weren't any to begin with, click the exit and your character will automatically dig to it.