Dead Island Definitive Edition

There are two methods to do this achievement , one being solo and one on multiplayer.
Solo Method *helps complete another achievement* :
- Run through act 1 with with the character you're working on. This will also help you unlock the "Right 4 life' achievement.
- By the time you hit the start of the second act, you should be anywhere from level 10 - level 16. If you need to get to level 15, just go around killing a few zombies.
- Once you're in the ballpark of level 15 or so, go to the arena. Buy a lot of Large health packs as you are doing this alone.
- Load up Arena "A" and start beating it until wave 15 then simply leave by exiting through the door or jumping off the map.
- Simply rinse and repeat this very boring strategy with the next three characters.
Multiplayer method, way faster :
- With this method, all you simply need is a friend with a character above level 35.
- From there, go to arena A and sit back while your friend starts doing most of the killing for you.
- After wave 15 is done, just exit the game and come back with the next character you need to complete this achievement.

If you're anything like me, you're a terrible gamer and this would be infinitely easier if someone just ran you through. But after a lot of searching through supposed video guides, I finally found one that made this rather easy.

What you'll need is enough molotov cocktails to be put on a government list. With all weapon slots unlocked, I kept two slots for actual weapons and the rest were stacks of 30 molotov cocktails. You can trade five alcohol bottles to Mike on the top level of the Lifeguard Station for a single molotov cocktail. Not a great exchange rate, but if you're up for some duplication glitch exploitation, you can ruin the thrown weapon economy with a surplus of molotovs.

A quick crash guide on the duplication glitch and precisely how I did it.
1: When you load in, go die. Find a zombie, jump off a cliff, if you're in the Lifeguard Station you can jump off the roof to the beach below. Once the "respawn" screen comes up, before the timer runs out, pause and select Load Last Checkpoint. Sometimes it throws you somewhere other than where you loaded in, so this is to make sure of where you will be reloading to.
2: Once you know this, back out to the main menu using Y on the pause screen. Sometimes things don't work properly if you try using multiple Load Last Checkpoints without reloading the save entirely.
3: Loading back into your save, drop your molotovs at your feet. You can do this by going into the menus with the map and inventory. Move over to your inventory, scroll down to molotov cocktails, keep pressing Y to drop them all.
4: Go die again. Once on that respawn screen, Load Last Checkpoint. You should respawn where you dropped the molotov cocktails. Pick them up and check your inventory. You should have double what you had initially.
5: Quit out to the main menu.
6: Repeat 3-5 until you have about 150 molotov cocktails.

Be aware things can get a little glitchy if you drop too many at once. I found it was best to not drop more than 80 items at a time or things disappeared. If you have 150 molotovs and a couple of good weapons, you'll be in good shape. If you can afford it, stock up on 20 large medkits which can be purchased in the Arena Lobby.

The crux of the strategy is to go to Arena A, the left door on the top level. You're going to camp out in the area by the work shop. If you're sufficiently leveled, you can get by in the first few levels by just kicking (LB) the enemies as they come at you, but you can just come out molotovs blazing if you want. You're going to throw the molotovs on the ground at the intersection where the bridge over the chasm meets the hallway to your right (as you face the bridge with your back to the workshop). Every zombie will be funneled through those two paths. The lower level zombies will run through the fire, die because it and then be on fire themselves.

There does seem to be some explosive canister in this area that will go off and insta-kill you. Be aware of that, but it's a one time explosion.

Some will start to get through and you'll need to strike them to finish them off. The bodies left on the floor will re-ignite when you throw a molotov, extending the fire walk they must go through to get to you. There is one enemy that will not be affected by the fire, and that is a Floater (big fat guy that vomits at you). You'll have to go up and strike them. Fire will often explode the Suicider before they get anywhere near you.

It might start to get crowded with all the dead bodies and fire you've thrown down. As this happens, go down the slope to your left (again, left assuming your back is to the workshop). If you have to go down that slope, there will be zombies coming from both sides and you'll need to throw molotovs in both directions. At the end of a wave, reset yourself to the workshop area method and only retreat down the slope when things get too hot there.

Be aware of the icons on your mini-map, watch for Suiciders and Floaters, keep an eye on your health and heal up when you're getting low. It might take a couple of attempts to get it all running smoothly, but I did it first try with two of the characters (and in the process of learning, blew myself up a couple of times with the others).

This is not my video, but it helped give an idea of the strategy:

Hope this helps everyone.