Protoss have fairly strong and tough units for attacking. But, these units are also expensive and can be outnumbered by the enemy.
A good strategy is before an attack, build a pylon just beside the enemy base where the enemy cannot see. After the pylon completes, construct 2 to 4 photon cannons and a shield battery or two.
Then for the attack, use leg-enhanced zealots backed up by dragoons to the enemy base. You can also use dark templars for sneak attacks. Do as much damage to the enemy as possible then retreat a little by little towards the outpost. Recharge shields, if the enemy chases you, the cannons will toast his units and the shield battery will refresh your units. Build more shield batteries and cannons if the enemy is stronger. Build more pylons and start cannons twoards his base interior.
You can also use speed upgraded scouts and carriers for hit and retreat and recharge tactics. The air ships will be of full health when they reattack. Zerg overlords can be killed this way, if the scouts are attacked and damaged heavily, retreat to the shield batteries.
Archons and multiple shield batteries can be very powerful if recharged and attacked with care. Your ouposts will serve as refueling stations and defense battle installations on the field. It is also effective to construct outposts first before expansions in a resource spot.