First phase he attacks you by biting, clawing, jumping and whiping with his own tail.
Second phase you will be defeating him much slower due that allosaurus require lots of bullets and he attacks by spitting flames and fireballs, firing 1 target instant death laser and jumping on the rock it protect you.
Third phase is easiest. Severals bullets defeat him.
final phase is hardest. Lots of bullets to defeat him. He also invisiblty to grenade
Shoot him in the eye for most damage. Take him health much per wave. Shoot the pteranodon around you and damage incoming t-rex.
Rinse and repeat. Do not allow your gun to overheat, get a bite from t-rex or get hit 2 time in a row too. Use these do defeat second t-rex and spinosaurus boss battle.