Try simple. There's a fucking Sea Wolf, ten feet from the party. In the time you put it down, it's put its teeth into at least two crunchy party members. So, you tank it. Problem being, you're now facing it solo, because a bunch of Blood Kites, or its mate, show up. Nature loves to fuck with people.
You're trying to grab some hot RnD from Ares. So is a party full of Shifters, who want to use it themselves, for whatever reason. Having some animal nature to them, they're almost guaranteed to scuffle over their objective. If they aren't the same animal, or aren't pack animals, they'll work separately, fighting 1v1 and trying to separate the party to make things easier for themselves. Since it's a rival team, it's totally within mission parameters. Considering the average Shifter mindset, (Not without exception, obviously) of Meta with a touch of 'beast' lurking under the surface, this would be the perfect situation for them to lose it to the animal within.
No need for illusions or doppelgangers. Just set up a tactical situation where the best solutions aren't always the ones you pick. Yes, it would be best to stick together against the first Sea Wolf and dispatch the second quickly after, but no, that's not going to happen in that situation. Yes, it'd be a brilliant idea to get the Shifters caught in their own lines of fire, but no, they're not going to get close enough to each other to try, because they're loners forced to work together by circumstance.