How do I complete the With Friends Like These achievement?

This is best done on Normal mode with a Hell+ character. Make sure you're not wearing any thorns gear, though.

Cathedral level 4 is the best place to try this. What you're looking for, though, are particular mobs. Bats and skeletons are good target, as they move fast enough to stay in a nice ball, and have low HP... but what you really want is a particular 'dungeon piece' that consists on a circle of bones on the floor, surrounded by four Tomb Guardians.

Once you've rounded up some trash mobs and your Harvester, kite them to the bone circle and let the guardians summon reinforcements. This should easily get your trash-ball up to 30+ mobs. Once that's done, you just need to pop the Harvester. For me, this was actually the trickiest part. I actually switched my LMB to the basic attack ability so I could pick off anything that was in my way without shredding half the pack with AoE.

I would recommend following this video!

I did it in the Weeping Hollow (act 1 normal) with a Wizard Hell+.

Just unequip all thorn gear, kite mobs (there are tons of wretched mothers that can summon up some more zombies) and of course the abomination harvesters. Run around until you find at least one harvester, then you can begin collect zombies.

Switch all your skills to defensive, so you won't have any offensive or aoe spells and accidently kill your horde. Equip yourself with magic missile, time bubble, ice armor extra slow, and teleport. Those non-damaging spells is supposed to aggro those around you so they move closer to you.

This way you can just let yourself getting punched while the mobs gather around you. Pop a time bubble to see how many you have gathered. If you barley fill the time bubble area with your horde, then you have enough for the achivment. Now from inside of your gathering, teleport next to where the harvester is and use your magic missile to kill him. He goes pop, everyone else dies, and you get the achievement.

The only time comsuming is to gather them all since they walk so slow, and the bloody harvester knocks you back. Good luck.