Having attempted this achievement numerous times, I think this is the easiest way to get it.
I used the Eyelander to move the bots, then switched to the Ullapool Caber for the actual killing.
NOTE: I DID THIS ON A DEDICATED SERVER, WITH A PLUGIN THAT ALLOWED YOU TO CHANGE YOUR LOADOUT. It should be possible on a non-dedicated server, but you'll save much time by doing it with a dedicated server.
1. Create a map on itemtest. Join RED.
2. Repeat this command 5 times: sv_cheats 1; bot -team blue -class pyro; sv_cheats 0
3. Make sure all of the bots are in a concentrated area. You can do this by hitting them with the Eyelander. Make sure not to hit each bot more than twice.
4. You need to reduce their health, so hit each bot twice. Make sure you don't accidentally kill any.
5. Charge towards the bots and hit them once with the Ullapool Caber. If you did it right, they should explode and you should get the achievement.
I will send a video of me doing it if anyone would like.