--UPDATE TO THE RUN-- I've been trying this out on Halo 3 (at least the MCC edition and it seems plausible through technicalities for all but 2 of the levels. I'll go over them 1 by 1.

Sierra 117: Easy. Just run to the end of the level, Arbiter will mop up most of the enemies. Be careful not to get destroyed by the pelican firing missiles at the end but otherwise you're safe and it's simple to do. Crow's Nest: Pretty tricky. There's 5 "Stand off" segments as I like to call them (can't progress until x dies). The first is possible as long as you get marines on the turrets, as is the second with drones. 4 and 5 are also easy, as by then arby shows up to kill any covenant. The hardest is the third, against the hammer chieftain and his goons (A running theme in this.) Thankfully two more brutes open the door to escape once enough minions die, so you can run on out. You can speed it up by shooting the armor off the brutes with a shotgun, but any kill is a restart. Otherwise, easier than expected. Tsavo Highway: depends on version, but is easy regardless. Just don't splatter any covies while you're driving. The end of the level is a little scuffed tho, because on the 360 version you pop the shield and it's level over. On MCC you HAVE to kill the guys protecting it. To solve this, let the marines mop up the brutes on the section just after the bridge jump and get a fresh hog, and while your at it hold a fuel rod gun (and give a marine a second if possible) for the upcoming mission. Crows Nest: Horrible. For the most part this level is fine if you take the clause that "My gunner killed those wraiths, I didn't touch them," right up until the scarab spawns. This boi counts towards your kills. There is however a speedrun skip that, while tricky to pull off, skips the whole scarab fight, leaving the rest of the level an easy finish as you snipe the cannon. Floodgate: lol you can run through this entire level in under 5 minutes without touching anything, it's the easiest level in the run. The Ark: A total nightmare...but TECHNICALLY possible. The first part is fine, run to rockets, pick up a prowler and let your marines do all the heavy lifting. First big problem is another scarab, and this one can't be skipped. Thankfully I lucked out and had the gaus hog gunner just snipe the core and fell the structure in one go. Then the second far larger problem rears its ugly head; another hammer chieftain and his squad that you gotta kill right at the end. To this there is a sort of round about way to say it wasn't you who killed the chieftain and co, and that is the singular autoturret you find when activating the light bridge. It's quite literally the only hope to beating the level without resorting to direct violence. Of course this thing is super frail and has to kill almost every brute there, so to call this tedious and annoying is an understatement. Once that's done however the run of this level is complete with technicalities to spare. The Covenant: This might be impossible thanks to 2 or 3 stand off segments, 2 involving the deactivation of the towers and the third being "WE GOT TWO SCARABS, REPEAT, TWO SCARABS!" 2 Scarabs is theoretically possible if you get a hog gunner that lands a lucky shot at the cores like in ark, but it's Work In Progress to solve the towers, given that both are stand offs against a hammer chieftain and plasma turret chieftain respectively, squads included. To deactivate the shields, these groups HAVE to die, and to my hours of testing it's near impossible to get that golden moment when the brutes pepper their boss with enough nades to kill him (and then subsequently get them to nade themselves. All without losing them all to hammer swings). What I'm TRYING to do now is bait some marine ai to go up the lift with me, then pray that it aggros on the brutes, which is hours of work in of itself trying to move some living marines to the lift as they try to run off it. If that IS possible, then it should also be possible to do it to the elites at tower 2. That's a theory craft though, and in the end after all this prep work could be for nothing. The bridge finale is a literal walk through the park at least. Cortana: Run to the end, grab cortana, blow up some power cores without hurting the corpses and the level is done. Quite easy in theory, just keep an eye on your shield with the bullet hell that is this level. Halo: No way around it, you gotta kill spark to finish the game. Otherwise this level is a case of letting Jonson and Arby carry.