I was bringing my lower level characters along with my level 72 commando when farming because the siren has the inventory maxed out at 39 which makes her an awesome storage mule. I killed the warrior probably 20 times, the bunker a bunch of times and then realized how much XP I was building and it backfired big time. I’ve got a level 58 siren facing level 58+ enemies with mostly level 53-55 weapons and a level 50 blockade shield. I thought that maybe the cheat would work in reverse; start a game as the siren, 2nd controller in my commando and blow through the levels. Well, that didn’t work. Everything immediately leveled up to my commando. No farming for level 58 weapons because everything was dropping 70-72 green junk. In short, my siren’s game is pretty much unplayable now unless I get some shift codes. I can’t kill anything with these weapons, can’t get new weapons without killing stuff, can’t get to chests without being blown away by enemies and can’t even use my higher level characters to get help. Both characters are in UVHM so I guess that’s why. Well, I’ve got a basically useless, 39 item, level 58 mule siren now. Damn.
Sad times… your best bet might be looking for a kind soul, in the online play/trading section for your platform, who may be willing to donate a few items.
I always thought a game was locked to the level of the host character? Can you survive long enough to get to Liar’s Berg and buy some on level guns from the Marcus Munitions vending machine?
There are also a few red chests that are farmable without having to fight much so you could visit those.
Tundra Express, Opportunity and Vault of the warrior all have a chest that can be reached quickly. Tundra you would have to run through some varkids Opportunity you just stick to the left border of the map you won’t aggro loaders and can farm the chest. Vault of the warrior you’d probably need your Axton to first take down Hero’s pass but then that’s also combat free chest farming.
I ran into a similar problem so this was my way out. I’m not sure how far you are in the story but otherwise I’m out of ideas.
When I use storage mules, I just bring them in to pick up or drop off gear, so I don’t have the leveling probem. I’ve posted this elsewhere, but the easiest way to get on level gear is to restart your playthrough and go through the Scarlett DLC as the enemies at first are not that tough (re-done bandits as pirates) and the gear you can get is very good. Within a few minutes you can get an Orphan Maker, but more importantly you can soon get a sandhawk, jolly roger, and pimpernel after just completing the regular missions with a few easy side quests.
If you need something right away, just go farm for a DPUH from the Torgue vending machines, you don’t even need a perfect one, a Hard or Intense Harold with any parts is still a very strong gun that will at least let you go through some missions to get better gear, As far as shields, if you have characters with extra crystals, you can get an Antagonist from flamerock (bring in the higher level char after checking the vendor, it will still be on the lower level if so) but I’ve found any good roll absorb or elemental resistant shield will do the job at those levels, they can be red chest drops so you can do stuff like Frostburn early in the game or that lower one in Sawtooth depending on how far you are into the story. There’s some relatively easy ones to get in Wurmsater and Oasis if you are doing the Scarlett DLC as I suggest. edit I somehow forgot to mention the Mercenary Day DLC and the snowman chest. You can run the “bring in the higher level char in the middle of the fight” cheese to beat the snowman (then remember to immediately have the higher level character leave, so the gear drops will be on the lower level character) and then you can farm the snowman chest for shields. There’s also a good chance at an awesome plasma caster and/or Leg Siren mod (or maybe just a simple non leg binder mod with a wreck or reaper buff) to drop from that chest as well.
I used the fast travel back to the southern shelf and got a blue slag smg from the gun machine. I was using a green turtle shield I bought from the machine and good god, this slow recharges slowly. I’ve been using the commando with a legendary ranger class mod that boosts shield recharge and decreases delay time … going to a siren with no real shield skills and garbage shields that take nearly 10 seconds to fully charge is tough. I wasn’t sure how far I was but my story mission is frostburn canyon. Brutal. I haven’t even met Lilith yet. I have 10 golden keys but I feel like it would be a waste to use them on level 58 when I could get level 72 purples that I could use long term if I like them. I hate leveling up and having to sell or store good weapons when they become ineffective. I always seem to have at least one point in every playthrough where all of my weapons are garbage.
Golden keys might do you more good now, as on l72 you can farm for much better stuff than you get out of chests and, if you don’t intend to do OP levels, you are at max level then. Do you have the DLC’s? You can farm Buttstallion and the Snowman chest for purples just as easily as with golden keys, plus they will occasionally give you legendaries, unlike the golden keys.
Some other things you can farm for in Southern Shelf/Windshear Waste would be a Hornet from Knuckledragger and a bonus package from Boom Bewm, though I’d be looking for a slag grenade of some sort (transfusion ideally, but bouncing betty works) as I prefer to let my grenades slag and my guns kill. The bonus package is a solid grenade mod, however, especially as a stop gap to better stuff.
Green turtle shield with Maya… yeah you can do a lot better than that. She has two shield skills on the left tree, Ward and Inertia. I use Ward with almost every build and 4/5 in Inertia with my main build as Derch taught me how useless Accelerate is most of the time and I need those points to go further down the tree for my normal Binder based build (that’s based on L72/OP8 though, you probably would ditch Inertia and go with only Suspension to get to Converge). Regardless, I wouldn’t use a turtle shield with her. At Frostburn, there’s two red chests to farm (one behind the barrier you have to deactivate and one nearby on top of the ice shelf above the starting area) plus a normal chest. You can just run through with dodging to get them. You can also grab an on-level Lasceaux (in a pool of water after you follow the left passageway when you go into the cave, which also happens to be part of the route to those two red chests) which is a fairly strong SMG and will get you through to better gear. I’d be looking for a blue or purple absorb or elemental shield.
I would go with the aforementioned strategy of snowman dlc snd exiting your axton right after killing him. The train will then be at your lower level maya. I have done that several times for Gaige and it was great!
I got a heap of money from being a mule for level 72 farming so item of the day weapons and stuff are totally doable but they’re mostly green junk. It took me a minute to remember which barrier I had to deactivate in frostburn but I know now. Where Zed’s monster hangs out. I got an outstanding Etech fire smg from the chest on top of the hill once. I’ll run around there for a while. I don’t have the snowman dlc but I’ll have to check it out. I got hallowed hollow and torgue a couple weeks ago. Hallowed hollow sucks.
I would recommend Mercenary Day DLC as the easiest reliable source of loot in the game (maybe the loot midgets in WEP)… The chest drops class mods, launchers, eridium, grenade mods and guns all with chances of good drops with occasional legendaries…wattle gobbler has red chests at the end which are easy to farm after, but don’t reliably drop good gear, hallowed hollow has Clark who will drop legendaries, but he’s tricky to spawn (guides on YouTube)… Scarlett, Tina, Hammerlock full DLC’s have good loot, Torgue less so but Harold’s and easier crystals, Blufatal would also recommend the Flakker 
I will have to get a few more dlc campaigns. I got Clark to spawn in HH but he dropped a pyrophobia launcher and I didn’t see any practical use for it. I have a purple bandit launcher that I like a better. I can’t remember if I sold it or gave it to my mule. Pretty sure I sold it though. Is he a world drop boss or is it just that gun usually? I used a keys last night for my siren but didn’t really get anything good. Ever open the chest and it has 2 of the exact same class mod and you don’t want either one of them? It was a gunzerker mod that increases pistol damage but decreases accuracy. Rubbish.
Clark just drops the Pyrophobia to the best of my knowledge, although to be fair I’ve only played the dlc twice. Most folks that like this dlc like it for the fact that it seems to have a high chance of tubby spawns…
I thought Clark was a world drop boss, but I don’t know for sure. Incinerator Clayton drops the Pyrophobia normally, but you’re right that it’s not fantastic at higher levels. Bad luck on the golden key drops, Mercenary Day DLC should sort you out and it’s only a few bucks and a fun level on top of that. You’ll get a useable Cat, Binder, or Trickster mod within a few runs and there’s a solid chance of a Leg Siren dropping, plus the occasional good drop on a gun, grenade or relic, the variety is very good and you can build up some eridium too. We’ve all gone through stretches of mediocre to weak gear on the road to op8, it won’t be that long till you get something useable