hmm you're having issues with tigrex but not kusha? That's interesting.. but anyway just take it slow with him. When he gets enraged it's not a good idea to recklessly jump and and try to get as many hits as you can. Wait for him to do an attack then he'll pause and you can get a few hits in, then roll away. if you're fighting him in the tundra, try to catch him in area 8 (the top-most one). Stand near the back wall to get him to run at you, when he does roll out of the way in time and his mouth should get stuck against the wall if done correctly, you can then proceed to attack the hell out of him. Traps are also nice if you're really having difficulties; and flash bombs are ok, but be careful when you use them - he becomes very unpredictable.
Focus on his back legs as that'll trip him quicker and also give you an easy roll away opportunity as you're not very obstructed.