A cross support team is all about having a group of specialists that can cover for each other.  Much like a good team has members with resistances that cover the weaknesses of teammates, a cross-support team has specialists that can handle the things that stymie their allies.  Walls have a hard time breaking walls- so they can be covered by sweepers or tricksters.  Tanks don’t handle sweepers or walls very well- in come the tricksters.  Sweepers may be stymied by the wrong wall or a trickster- here comes another sweeper, a tank, or a trickster ally.  Tricksters are too fragile to handle some sweepers- so you have a tank or a wall on hand.

There is no magic combination- I could cry ‘two walls, two sweepers, one trickster, and one tank’, but that formula could be completely wrong if the wrong pokemon are chosen for it.  What exactly you put on a cross-support team has to depend on what else is on the team- if even a single slot can’t be filled with the pokemon you originally wanted for it, you have to take a close look at the rest of the team members as well as that slot, and determine if what you substitute in needs a different form of support.

Cross-support runs deeper than that, though.  A trickster wall like Umbreon can Baton Pass a good situation onto any other wall- making it a good wall to have with your offense-capable wall, or to pair along with a tank that can handle the Bug and Fighting pokemon that threaten Umbreon.  If your cross-support team includes a Snow Cloak pokemon, your tank may need to be Abomasnow, just to get hail out there (not that Abomasnow has no merits of its own- just watch out for fire attacks).  Having a lot of pokemon on the team with weakness to fire may be a call for one of your pokemon to have Flash Fire and switch in at opportune times.  Use Leech Seed and swap to Dusknoir.  Fight for a little with your Ninjask and then Baton Pass it’s speed to a Rampardos.  Have your Vaporeon use Wish and then swap to an injured Forretress.  Use the last few HP on your Tropius to lure out an Ice pokemon- and then crush it with a Mach Punch from your Breloom.

Cross support takes all these forms and more- any pokemon that can put another pokemon on your team into a good position is cross-supporting.  Any pokemon that can jump in where a teammate is weak and be strong instead is cross-supporting.  The best cross-support teams strike enough of a balance that the turn lost swapping pokemon is more of a help than a hindrance, no matter whether your opponent attempts to press through your advantage or swap teammates themselves to try and turn the tables back.