I never said that it can rangespam, as its style is mostly through 2 methods of jump attacking in most of the games it's been in. However,
some variants do in fact lead with bullets, and considering how this is a composite Slime... yeah. Even if this is going to turn into a full-on close ranged battle, the T-Rex won't be smart enough to keep its distance either, which is terrible as the Slime has a slight AP advantage.
The issue is that
the T-Rex cannot attack while it is turned into a slime, and on top of this, it also takes twice the amount of damage while turned into a slime which will make it go down faster.
There's still the issue that the T-Rex is slightly weaker than the Slime, not to mention how it has fewer abilities to work with. If anything, the Slime having a smaller size actually gives it an advantage because it would make it harder for the T-Rex to actually hit it with its attacks, and Slimes have been competent enough know how to dodge arrows in the past. Comparing their physical attacks, the T-Rex has nothing to inflict with its attacks (Apart from pinning the Slime down which it can either dodge or teleport away from its grip) while the Slime has more attacks to dish out and is slightly stronger. Even if its status chance is low, it's better than nothing as both of them are nearly even in AP and the battle would last long anyway for Slime to land at most 10 hits for a guaranteed chance of one of its two statuses landing.