The Protoss defense is quite simple. You just have to have Photon Cannons all around the perimiter and have observers patroling around. The Protoss also can build up more than one base faster than any other opponent because they warp their building from their homeworld, so they can set down the warp beacon, and start gathering more resourses while the building comes in by itself.
The best attacks are to have lots of Dragoons and Scouts because they inflict a lot of damage and they have high hit points. The way to attack with them is to come in from different sides. (if attacking Zerg with this attack, you want a backup group so they don't regenerate.)
My favorite attack (which has never failed) is to get four or five Arbiters to just fly into the enemy base. After that, you recall more than 12 Archons (and get observers too) and attack. The Archon is the most powerful unit there is in StarCraft (based on other units, not heroes) so you could tear everything apart and only loose a few units. If you are fighting Terrans and they have a Science Vessal, use your Arbiters to put a stasis field around them so they can't use the EMP Shockwave on you.
For air attacks, you need three or four Carriers all full of interceptors, and attack the outer rim of the enemy base. Next have lots of Scouts, Arbiters, leftover Carriers, and Observers(if you're fighting Terran) and attack. If you have one Arbiter left, then recall any friendly units and use them for the attack.
Any brute force can be used when you don't have a good attack plan or you don't want to use any of these strategies.
A strong ground attack uses every ground unit. First use Shuttles to carry Reavers over close to the enemy base. Next, get lots of Zealots behind them. Then, you should have Dragoons behind them and Archons behind the Dragoons. Now you desroy their defense with your Reavers and Zealots, and have the other units right behind them. Keep them ocupied with the Reavers untill they get destroyed THEN!, you attack with the Dragoons and Archons.
When fighting Zerg, you want to use the airial attacks because not very many Zerg units can attack air units. When fighting Terran, use some brute force in the begining, then use a strong attack like the Archon attack or the all ground unit attack.
If you have any other great strategy, e-mail me at: cho_gall40@hotmail.com