I think it's totally fine to self-insert a character in with the group, so long as you do it with having fun but not simply gratifying yourself at the expense of players, but I think the most important thing you can do in an RP with an opponent for the group is flesh that person out first and decide what the guiding psyche is for the antagonist/opposing faction and work from that perspective. 

That then gives you the ability to more solidly determine what sort of methods the enemy uses and what they are less adept at dealing with. The players need to be aware that they need to adapt to this enemy.

[@mdk] and I are co-GMing an RP where we essentially settled down on who the enemy commanders were -- people that the players might not even actually ever see -- and how that would dictate (this is a Star Wars RP) the actions of the Imperial Navy operations in the region.

[@Trinais] and I are co-GMing a modern fantasy where, from the outset, the opponent is an ancient being of immense power...but for some reason he relies on subordinate henchmen to get things done. One henchman, a vampire with an organized crime background, was just replaced by another vampire that fought the US Civil War (notably, Sherman's March to the Sea) and was involved in the wars (massacres) against the Plains Indians in the 1870's. The crime lord was slow to marshal other vampires in the city to the cause because he was trying to take a cut of the profits of their operations. This new guy? Not so much. There will be a strong contrast in methods there as the characters driving the enemy's thought processes change.

So in a sense, I think if you're going to devise a strategy against the players then you need to devise the enemy they strive against and then make sure you're compartmentalizing what your character knows versus what the enemy knows.

To be fair, I work with good players that tend to already understand the difference between OOC and IC knowledge, so I let them see the poker hand quite a bit. That -really- helps. ;)