Here is what I've determined:
Varkids need at least two members in a tier for one of them to evolve to the next tier.
Some varkids won't ever evolve.
All that stuff about varkids needing to be damaged a bit, to half-hp, or near-death to evolve? Also a wild goose chase. Don't shoot them, don't kill them, except when you know it will help(I'll get to that in a bit).
All that stuff on youtube about having to get the Ultimate to evolve by killing things? A wild goose chase. I tried that. I tried that for HOURS. It didn't work.
If you're doing this solo you can speed it up by using the app that fools the game into thinking there are more players. In my experience the app does up the difficulty/number of spawns but doesn't make the loot any better. It does help in this one because more Varkids = more chances to evolve to next tier.
Your evolve population doesn't have to be all from the same camp. This is the trick to getting this to work when it seemingly won't.
Here are my steps:
1. Bump player count up to four, either with 3 more players or with the app. I had to bump it back down to actually kill the bastard and even then it took all of my shotgun, assault rifle, smg, and pistol ammo.
2. If you normally wear a Bee shield swap it out for a much higher protection shield until you're actually killing the boss. Bee shield will get you killed over and over again during the evolve stages. I have a ~75k shield and a +30% shield relic for this step. Even with a good shield I die 2-3 times.
3. Go to the farm, run a loop between the two camps in front of it, forcing all the larva to come out. There are two "camps" in front of the farmhouse. You want to keep both of them "activated". You can do this by running through them, aggroing the mobs OR you can just shoot the ground in the camps. Do not kill any of the Varkids unless you absolutely have to. Less varkids = less chance for final evolution. If you die you'll just respawn at the new-u under the farmhouse, no big deal. Inside the farmhouse and inside the barn are two spots of relative safety. You have to watch out for spit through the windows and doorways but normally nothing beyond larva will enter those buildings. I did get a super badass glitched inside the farmhouse once, though.
4. If you're at 4 players chances are 50/50 you can get the boss to evolve just with those two camps. Every now and then some buzzards will come in. They can help or they can hurt. Don't let them kill any varkids. Do let them provide some distraction for you to keep both camps activated.
5. If after oh, say, 15 minutes of this you don't have the final evolution you're going to have to drag most of the varkids to another camp and keep them there long enough to add the varkids from the new camp to group. I like the camp in depression next to the bandit house that is on stilts. You can hide under that house and then just shoot near their feet to keep them at that camp and keep the new ones there. I've never had this fail once you get to that camp.
This is the house I'm talking about. You can see it underneath the giant dead Varkid, lol.:
6. Bring lots of ammo, ammo regen, or an infinite ammo pistol. Since this guy is 53 he is a skull and consequently either has a lot of damage mitigation or an ungodly HP pool.
7. If you go to the third camp and no varkids come out it means you're at the cap for total number spawned at once. Typically this will mean you have a lot of badass varkids. Kill 2-3 of them and they'll get replaced by larva at least one of which will hopefully evolve beyond badass and be what you need to get the final evolve. You might also have 2-3 adults flying around, not evolving, you can also kill them and get replacements.
8. I never had to let the Ultimates kill other mobs. What I did have to do was have at least 2-3 Ultimates at once. It is remotely possible that the ultimates got some buzzard/bandit kills that I didn't notice but it isn't likely. I took an ultimate all over caustic caverns, letting it kill every thresher in there, at least when it wasn't stuck inside a barn or tree. What a pain in the ass. Don't do that to yourself.
9. When it comes time to kill him do not forget to put your Bee shield back on. I did this not once, but twice, lol.
10. I can't give many tips for fighting this guy straight up. I shot from cover 100% of the time. It is cheesy, but effective, particularly since parts of his body will often clip through the cover allowing you to shoot him without leaving your cover. His ranged attacks will knock you down if he gets a direct hit. I'm pretty sure he has a bit of a charge similar to the badasses as well. He shoots bug missiles like the Ultimate does, seems like you can get a 2nd wind off of those.
P