Minecraft Dungeons

EDIT 4: A little side-note that I mentioned in the comments that I forgot to mention here is that the tridents still can detonate even when hitting mobs. The only problem is that it acts similarly to the Fuse Shot enchantment and doesn't detonate if the initial piercing kills the mob, which is just kind of annoying. So either aim for a wall like mentioned before, or aim for a beefier mob that'll be able to tank at least one trident hit.
EDIT 3: Although the achievement is perfectly functional and obtainable, the devs changed the behavior of the tridents. When you throw them directly at a mob, it inflicts only the piercing damage without detonation; however, they detonate when they are thrown at a piece of terrain and seem to do less damage than when I first completed this. As recommended in the comments, you should probably farm for a few tridents just in case the initial explosion doesn't kill, and you'll probably have to stand in the blast radius to ensure the mobs stay within the area.
EDIT 2: They fixed this achievement with the latest update, so get skewerin'
EDIT: Note that several players are having issues with unlocking this achievement following the recent Halloween update. Despite meeting the requirements, the progress bar gets stuck at random percentages, so you may have to wait for a patch before this achievement becomes obtainable again.

So how I went about getting this achievement with relative ease was by going to Radiant Ravine and progressing through the level until I reached a room that exclusively contained tropical slimes (this room isn't a guaranteed encounter, so you may have to play repeatedly to get it to spawn). As you go through that room, it'll eventually get to a point where it spawns the slimes infinitely. You can either kill them until one happens to drop a trident, or you can keep one on-hand until you find the room. It's probably worth noting that I don't recall any bubble columns being in that room, so you may want to just keep the conduit from the beginning of the level on-hand and just farm for tridents in the room. Anyways, you'll want to kill all of the big slimes so they split apart and then group together at least 10 small slimes. Throw the trident at them while they're grouped up, and hope for the best. If it doesn't work the first time, you can continue farming for more tridents and just group up some more as they respawn.

tl;dr - Go to the tropical slime room on Radiant Ravine and throw a trident at a group of 10 small slimes.

Tridents are dropped randomly by any type of mob on any of the new levels. They work similarly to the TNT, as they take up your arrow space and are a thrown item that explodes. The big difference is that they attached to whatever they are thrown at, including mobs. Play any of these levels on a super low difficulty and hope that the RNG spawns a Trident. Then just allow a whole bunch of mobs to group around you and throw the Trident into the group for a ton of easy kills.

Go to the mission Coral Rise on default or adventure. Run through and collect a spear off a drowned zombie. When you get to the drowned necromancer many mobs will spawn. Group up the small zombies and rest of the mob and throw it, boom achievement will pop!