Alright, first off - I'd like to preface this by saying I'm a little frustrated with myself. I was working on 'this' post last night, closed my computer, woke up this morning and started moving all my stuff to the my college apartment, which is a three hour drive from home. After finally unpacking everything, I finally got back to writing this and accidentally closed the window, losing a good page of material. -_-
On to the topic - playing disruptively. When most players write an army list, they usually have some sort of game plan in mind, whether it's to fly around and pick up the win/tie on turn 5 or to throw enough kill your way to table you by turn 3.
Whatever their strategy is, your strategy becomes to counter it in a suitable fashion, making their tidal wave of bloodthirsty killers wash harmlessly against the beach, and then laugh maniacally as you kill it dead.
The first (and probably most important step) is to look at your opponent's army and to understand the role of each unit in the army:
1) What can each unit do (alone or with support)?
2) How much damage can take before it's relatively ineffectual?
3) When/where is it strongest (early game, late game, in your face, at a distance, in cover, out of LOS, etc)?
The faster you can do this, the better. No one likes to be kept waiting for 20 minutes to start a game just so you can plan a Machiavellian victory.
Then, ask yourself these questions in order to gauge your weaknesses. Is your army particularly vulnerable to shooting or assault? Is it mobile or static? Is it tough and resilient or are you wearing tissue for armor?
When you're done, prioritize the enemy targets. Your goal is to nullify your opponent's strengths, while simultaneously covering your weaknesses.
1) In just about all cases, I make it my first priority to cripple the opponent's ability to respond to you by wiping out his transports (or if there aren't any, then his other mobile elements). Turn one, I leave only my durable units on the board, while the fragile elements usually stick back in Reserve.
2) After his mobile elements are either crippled or destroyed, your goal is to destroy or nullify the greatest threat to your army. If it's a shooting unit, assault it or use some LOS creatively (Rhino walls are a common tactic) to try and negate some hits.
If it's an assault unit, you have three options. You can either shoot enough bullets at it to make it a nice pincushion, give it a speedbump, or ignore it. The key to a speedbump is that your sacrificial unit (volunteers please!) HAS to die in the opponent's charge. That way, you're free to shoot the snot out of him next turn. If he kills you on your turn, then his assault squad can continue to hack n' slash through another unit. The last option is harder for many armies to pull off - you have to either negate their speed or just be naturally faster than them and play a giant game of cat and mouse.
3) Play Clean Up - On the last two or three turns, you can consolidate your objectives, mop up any other targets of opportunity and play conservatively. The faster your army is, the later you can jump onto objectives.
In a nutshell, that's your battle plan. Assess, Destroy, ????, Profit.