Briefly, the secret to defeating opponents quickly in 4e: create windows of advantage, and go through them as quickly as possible.
Example: use action points and dailies in two circumstances. First, use them when you have a temporary advantage. Am I playing a ranged character, and does my target grant me combat advantage? That won’t last forever. Unload now. Bonus points if I understand the capabilities of my party and I know that sometimes I’ll get combat advantage plus a bonus to hit. Then I can wait for the +4.
Or is the monster nearly dead? Does it go right after me? If I can kill it now, I reduce its options in a rather dramatic fashion, so let’s do that. Maybe it’s worth spending an action point to finish it off, even. If half of my party gets a turn before it goes, well, then the impact of my extra attack is not as large, relatively speaking. The window of advantage is longer.
GMs can play this game too. People will tell you that a high level cleric can outheal anything your monsters can do. That’s nice, but clerics get to act once per round just like everyone else. Start your focus fire with the monster who goes after the cleric, and see if you can drop the target before the cleric gets a chance to do anything. Rinse, repeat.
Or just focus fire on the cleric. The defender may do something to you as a result. Big deal. Her AC is 3 points higher than the cleric’s anyhow, so you’re more likely to hit the cleric than the defender and you’ll do more damage to the cleric expressed as a percentage of the cleric’s hit points. Use your windows. Open windows for yourself. Sure, the swordmage can blunt one attack per round, but only one. Open the window by using up his immediate actions.
Disclaimer: in practice I always wind up using my action points and dailies because my PC is pissed or enthusiastic or whatever. My PCs aren’t usually as smart about tactics as I am. (Which is a low bar anyhow.)