I believe it's due to the lack of real power in your weapons ATM, Ignore the quest for a while and try to work on Blangongo and make the ice katana from it' part, the monoblos is weak to ice and lightning and the Ice katana packs a punch while sporting a healthy chunk of both green and yellow sharpness.
I would suggest you work on weakening it with bombs if the monoblos poses a problem, make and bring both the shock trap and pitfall trap as well as bring trap tools and materials to make shock traps, 3 combo books(book 1,2 and 3), flash and sonic bombs, the materials to make both bombs. The G bombs(large bomb + scatter fish you can get in pokke farm when you upgrade the pier) large bombs, paintballs and as many healing items as you can bring.
Tip one.
DO not use a flash bomb needlessly! The monoblos is sensetive to flash bombs but like the diablos, almost always screams multiple times when it is blinded, only flash it if it's in rage and you are far away to not suffer it's screams. I flash it only when I'm far off and needed to heal, sharpen my weapon far away from it or when I'm simply protected from screams... you need a high grade earplug to effective shield yourself from screams.
Tip two,
Use sonic bombs when you are sure it's not in rage and you can get near enough as it burrows to the ground. giving it a sonic bomb as it burrows( time it as it has it's tail tip almost submerged) and you will have it trashing hapless for as much as 15 seconds. Not effective if it is in rage(you'll see the crest doted with 2 rows of red marks.)
Tip three,
do not approach it from behind as it springs out from underground, it swings it's tail around and it is still dangerous even if you had cut it's tail off.
Leave it's horn intact for as long as you can, since you already knew how to exploit it's charge by having it charge and get stuck onto walls or rock pillars, use this behaviour as much as you can. I almost always aim to cut it's tail 1st because you get 2 craves from it's tail and it has a smaller area of effect when it spins around to sweep you with it's tail.
Besides exploiting it's weakness to ice, you can also consider poisoning it with SNS weapons as I have mentioned before, you deal less damage over all but the poison effect sticks to it for about a minute and a half, the poison damage irrtates it and makes it come out of the sand should it chooses to burrow oftenly.
If you took my suggestion to use bombs on it, plant as many bombs as you can when you have it nailed down either from a sonic bomb while it is underground or from traps, ignite the bombs with either items you can throw or small paint balls.
There would be some other ideas I have such as spamming duals on it's legs but I leave the choice of weapons to you.