DeathSpank

This achievement obviously requires you to kill 2500 enemies.

I got this achievement about 75% through the game. If you are going for all the achievements then you should get this way before you finish the story. If you want to grind it out, though, the best place is probably in the haunted forest. Along the path in the middle of the map are a bunch of spiderlings. You can one hit them with just about anything so you can run through them quick.

If you press RB for equipment, then press it again to tab over to inventory, you can read your "book-o-stats" to see how many enemies you've killed. If you want to know your progress, that's where to look, for this, and other achievements (like the 50 side quests one)

Signal24Code20 left a comment saying that the chickens count as enemies, so if you want to lock this down early, you can simply kill all the chickens near the beginning and outhouse teleport to respawn them. Credit goes to him.

This one comes naturally as you progress through the game but if you feel the need to grind it, executing a perfect block to fill your justice meter and performing the justice attck with the cleaver of cleaving (lv -- the higher the better) on the spiderlings along the graveyard path as mentioned by blue slime

Alternativley, If you picked up/kept the repeating crossbow, running round killing stoopid chickens/varocious chicken will help boost your kill count too. But, as originally stated, I'd consider this natural progression and You should only consider grinding If you don't have it when you hit lv 20