This achievement unlocks after you get a combo of 50, which is immediately recognizable since
You can unlock this on the first world, and even unlock the 100 kill combo, assuming you get decent rng for your paths.
Loadout:
I'd like to give credit to vSully from the walkthrough since this was the loadout he recommended and is definitely the best loadout in the game. You're going to want to get the Shark Backpack and have the Scythe as your weapon, it would also help to have the brick head so you don't have to worry about collecting your stuff if you get hit.
Shark Fin Backpack Location:
Scythe Location:
*** Spoiler - click to reveal ***You can buy this in the first shop but to have it spawn as an option: you must spend all your glyphs on re-rolls in the first shop. Apparently it might take about a minimum of 10 re-rolls for the scythe to appear but once it does, you then have to get a 5 hit combo on the mushrooms on the roof to get the 1 glyph you need to buy it!
Brick Head Location:
Some pointers:
1.)Remember that once you start: your streak will be on a timer, don't try to take out a risky enemy if you're not sure you'll be able to make it to another enemy before the timer expires!
2.)The timer stops when you're inside the worms but as soon as you exit it will begin again.
3.)Bouncing on enemies keeps the timer going, use this to your advantage if you need to buy some time while an enemy is doing their patrol!
4.)If you still don't have this by the time you get to the level one boss: you can use the shark fin barrage to decimate the entire boss room in one motion, I would suggest doing that as soon as you drop down to ensure that the timer doesn't expire before you can land a shot on those pesky flying enemies.