For those that are here to simply get the achievement, your best bet is to do use the spawn dummy technique. It works best with a friend so you can actually control yourself and avoid dying. Otherwise, with all the extra spores around, your afk self will most likely end up dying at some point without anything you can do. So the idea is a "manned" dummy strategy. You have to be playing hard enough to kill the flood that may try to come at you as you stand as far back as possible, and your friend gets to fight and die for you. It's good to be king, right?
Note: According to other reports from other players, you *cannot* get this splitscreen, as doing will negate your victory if EITHER play dies. So to "spawn dummy" you have to do it over live, where it has been confirmed as possible by the player "Breadly Weapon". As well, you cannot reload after you die, the game will remember that you did die. So if by chance you notice yourself with 2 bars and no shields: reload your save.
Now, that is the easiest way to get the achievement. For those of you who enjoy the feeling of earning the achievement the hard way, for that "I'm awesome" feeling that many of us enjoy, here's my advice on how to do it solo, without co-op'ing it.
You're going to want to use a shotgun and pistol. After multiple playthroughs on different platforms, I find it the most effective combo. I mainly rely on the pistol, as you want to stay as far back as you can. You can use rocket launcher if you want, though I would advise against it, as it will often get you killed. This is due to the nades that fall from the flood as you kill them. If you shoot rockets, you may end up blowing yourself to hell.
Now, when you play the level, you're going to want to creep slowly, as you don't want to die. Move up until the flood spawns, then drop back. Pistol them from a distance. It takes 2 shots to make the spore carriers explode, and it only takes a couple more to down a regular flood. Shotgun is for when they get to close. After all, they swarm, so you will need it, but the object is to use it as seldomly as possible, otherwise the enemy is too close.
Be very careful when the spore carriers (the exploding walking bags) spawn. Much like rockets, killing one of these can make the whole map explode, ending your attempt. However, you can use them as free grenades. If you see one in a cluster of flood, shot it with the pistol twice, and the whole group will explode, landing you insta-frags. You also get the satisfying feeling of watching those guys die en-masse.
Always keep moving. You take minor damage from the flood spores, but in a firefight, it's enough to keep your shields from recharging. So if you stand still, you'll never heal, and you will die. So keep moving. If you're doing this without the bandana skull on (which, is very nice, though if you're doing this solo, I would assume you're trying to do it the "hard" way) I would recommend that after you kill the main combat flood forms, to simply melee the spores. If you're doing this with bandana skull on, scratch reading most of this tip-guide, and just roll with rocket launcher and grenades, stay back, and spam everything you have.
In essence: proceed slowly, don't stop moving, conserve ammo. Against flood, human weapons are your best bet. Don't try covenant weapons as they'll do little to no damage in comparison. If you throw nades, be careful, as it's easy to frag yourself from blowing up dropped grenades, and especially: DON'T USE PLASMA GRENADES. Why? Because if you stick a flood combat drone, they run, and they run fast, right at you. It's near suicide. If you're good with grenades, go ahead and use plasma, but try not to stick anything. Throw the grenade before flood spawns right beneath them, so it explodes as they land, or in front of you as flood is charging, so again, the nade will explode as they run by.
Also remember: no matter what you do, it's very easy to get randomly fragged. Keep an eye on your radar. Small dots can be ignored, those are spores, deal with them when you get the time. Big dots, those could be spore carriers, and they hurt. A lot. If you do happen to get caught by one: jump. It'll get you instantly moving, hopefully enough to get you out of the way of impending death. If not.. restart the level.
For those of you that enjoy doing things the hard way: Good luck.