I have finished Normal mode, and went through True Vault Hunter Mode until Sanctuary lifts off. I now have a problem leveling up an reaching Sanctuary again. My sub missions are all done, except for those I can not complete yet. I can't level up in True Vault Hunter Mode because the missions must be turned into sanctuary. I struggle to get the Fast Travel Beacon without dying, so I don't have sanctuary on my map. How can I level up quickly in this situation?
It seems to me that your key problem is getting Sanctuary back.
I'm not clear on where you're getting stuck, but it's probably one of two big fights:
1. The Gluttonous Thresher
There is very much a right way to beating this monster thresher:
Run out into the area where it spawns.
As soon as you see or hear the thresher, perform a rapid strategic withdrawal. (i.e. Run away.) You want to get back through the door and out onto the big industrial "dock" area.
If you're playing an Assassin or a Commando, use your action skill to distract the thresher to ease your getaway.
Once through the big door, find some cover. I use the big containers sitting on the left side of the dock as you exit the big door. (You could also cross the river on the retractable bridge and use the cover over there.)
The thresher will chase after you, but will stop just past the big door, not going past those K rail barricades to one side of the big door. This is the key to this strategy: the cover you're using is outside the range of the thresher's tentacle attacks, and it is the tentacles that make the Gluttonous Thresher fight so difficult if you fight it where it initially spawned.
You will want a shock weapon in this fight, because the Gluttonous Thresher has a crazy-fast shield regeneration. You need to zap the shield away before you can start knocking its health back down again.
2. The Robot Fight In Overlook
After you get the beacon, you're asked to take it back to Overlook where it's used to summon a fast-travel station.
This is also a tough fight. There's not as much of a single right way to fight this one, but again, cover is the key. I typically hide behind the grinder while reloading and waiting for my shield to regenerate. It's good solid cover that you don't have to duck behind to get full benefit from, and you need to be nearby anyway to repair the beacon several times during the fight.
This strategy doesn't work as well against the loaders as it did against the Gluttonous Thresher. Primarily, this is because the robots drop into Overlook at two points, 90° from the grinder. This means that while you're fighting one cohort, the other — which you're protected from — is marching closer to the grinder. You thus have to keep running around behind the grinder to pound on each cohort separately, lest one of them overrun the grinder, denying you benefit of its cover.
I've gotten a lot of use out of my Rolling Thunder grenade mod in situations like this. Any homing grenade can help, too, since you can just throw a bunch of them as you run behind cover, which keeps the robots busy while you recover.