Usually the best thing to do with it is keep on the sidelines, waiting for an opportunity when the opponent’s HP is low enough that swap-in stunning strike will take them out. Of course, if your dino that’s already on the field can do that anyway, it might be better to save Monolorhino for later. It’s usually best at sniping faster opponents. Then after swapping in, it can do a significant amount of damage to the next creature, and if they’re at the same level it is almost guaranteed to be able to wait out the swap-prevention and swap back out, and then swap back in again later, although it depends on the situation. Even the priority shield is useful to keep Monolorhino alive so it can be swapped out and saved for later.
If the opponent isn’t low enough to be taken out with swap-in stunning strike, but they can be stunned, then you can swap in, hope to stun them, and then finish them off or even stall and then swap back out. This is useful for dealing with creatures like Suchotator and Thylacotator, which can severely damage one of your more powerful creatures, but can do very little to Monolorhino.
And finally, Monolorhino is decent 1v1 against the right opponents too, so you can bring it out, deal some damage, and then swap out to save the swap-in damage for later, if you don’t take the opponent out then and there.