Here's a lil something concerning what we define with tough combo's.

Easy (And therefor most liked in PvP)
xNumber of the same class/spec, or two classes with perfect synergies (like priest/warlock). Ranged only. (perhaps with the exclusion of multiple DK-boxing due to the skills they have at their disposal. But they still require you to be on top of things.)

Moderate
Any set up with max 1x melee and the rest ranged healers/dps.

Hard
Managing multiple melee into a box.

Those are the three basic difficulties you will encounter. The rest is all based on how well your macro set-up is and your ability to see what's going on in a fight.