I've done the calculations already and posted them in the Discussion part of the Forum in the page "T-Rex: Long Live the King?". But, just incase you can't or don't want to go find that, here are the facts:
A Carnotaurus (let's say level 20, without pots) needs to inflict ~26 hits before it kills a Brachiosaurus.
However, a Brachiosaurus (level 20, without pots) only needs to inflict ~17 hits before it kills a Carnotaurus.
The maximum boost you can get from a single Damage Potion is 65% of your original damage (I'm assuming this is correct, because I can't find another source to confirm that my memory is correct).
This would temporarily increase a level 20 Carnotaurus' damage level from 29 to about 48, which would raise its damage per hit from about 87 to 95, and reduce the number of hits to take down a Brachiosaurus to ~19 hits.
Still, it's 17 hits < 19 hits, so the Brachiosaurus would still bet the Carnotaurus, no sweat.
If you were able to use a damage reducing or reflective potions, then maybe (just maybe) the Carnotaurus would have a shot of winning the fight.
OR if you account for your gun's damage output and you are fighting a wild, RIDERLESS Brachiosaurus, then you'ld probably have a fair shot at winning the fight.