A late answer but for anyone curious, once you are pulled in whether you don't know when it's coming and can't find cover quick enough or you just do not focus on the health, completely understandable, there are a few tricks you can perform.
Take note, this move often occurs around 15-30% health region. Terramorphous will also become extremely vulnerable to attacks at this stage as well.
Of course the character you are is the most dependent on what you choose to do, assuming you are level 48-50, otherwise just leave... anyway these are the best methods I could advise you to do whether it be solo or 4 player co-op.
Zer0: If not already adjusted, adjust Zer0's skills points into "Bloodshed" as deception mode is fantastic! Throwing Kunai is also a nice bit of help. Also wield a weapon, usually a magnum (Often called the Law?), or something like that with an under-barrel blade allowing you an additional 100% damage approximately. Once being pulled in activate deception mode early IF you have Kunai, and pelt with Kunai whilst ensuring you have the bladed magnum (Law?) equipped ready to being melee attacking, non-stop. Once touching Terramorphous unleash your deception mode enhanced, additional melee damage and skill enhanced melee attacks into him, bringing his health down rather rapidly, from the state it's in. In brief: Equip bladed weapon, slasher time.
Axton: Just ensure that Axton can throw down his Sabre turret and do your damn best to stand behind the turret as this can work like the rocks at the back of the area, helpful because the rocks can't hold 4 people, not even 2 in my experience, you kinda just keep fighting each other for the spot... Anyhow, keep blasting away while your turret acts as another player causing damage and hope for the best that you do not get pulled in too early, as Axton is one of this most difficult players to be at this time. In brief: Drop a turret, gun it down.
Maya: She is extremely useful throughout this entire fight keeping herself and other protected whilst making Terramorphous more vulnerable to all attacks, assuming the skill tree has been adjusted. During this wormhole part, using Mayas ability can actually reduce the damage taken from being in contact with Terramorphous, depending on what part of Terramorphous you are touching, but again, use the ability to weaken Terramorphous and spray her down. In brief: Cause giant bubble, pop giant bubble.
Salvador: Could there be a more obvious option with him? Spray, spray, spray, spray, spray and maybe roar while doing it. Your options are very limited with this character, for the simple matter he is a f@#&ing psychopathic animal, eating machine! Just make sure you equip two good weapon choices for this final moment, probably launchers with large clips size around 4-8 shots in each clip as this final part shouldn't take long.
In brief: Gunzerk, Don't miss
I unfortunately can't help any further as I have not used the Mechromancer or Psycho but would imagine they are of better use as DLC character usually have some over power form to them. Hope this has helped and if not, go talk to Claptrap, he will make me more persuasive. ENJOY!
