The following are attack combinations you can use in Skirmish missions:

Pitbull
Predator Tank
APC with a Grenadier Squad inside

3 or more Rifleman Squads
3 or more Missile Squads

2 APCs with Rifleman Squads inside
2 APCs with Missile Squads inside

2 Grenadier Squads
2 Rifleman Squads
2 Missile Squads