With any race you need to identify your specialists for: Expansion, Attacking, holding a square and taking a beating (Meat Shield).
For Orcs I'd say these units apply best in the order listed and funds available:
Expansion: Raiders, Mystics
Attacking: Raiders, Wyvern Riders, Ogres
Meat Shield: Mystics, Ogres
The thing is for attacking, you want to pair up your units to do perfect kill combos, or use one unit to kill another with just one hit. You can usually count on getting one or two guys in for any particular kill.
That's my take, anyway. I'm sure there's other strategies that might work better. Maybe focusing on early strikes via Second Wind to get close to the enemy, but then sacrificing expansion/getting more money, which would make Mystics a first priority for attacking.