Truth be told, I went up against my fair share of premades solo, and the one thing I noticed was that they cover each other, they mark and focus targets, and they stick together. . . You don't need to be in a premade to do these things, and on rare occasions I've gained victory over premades by having a random group that would operate in a similar fashion.

Don't be swayed or deterred by the fact that your enemy is in a premade party. If you know what to do, and you're confident enough to lead, step up. You could very well be that key component the team needs to win.