Hard counter against Terran as Protoss

The General Strategy:

This strategy is an early game strategy, designed to get you to the midgame with an advantage over your opponent using stalkers and void rays. After that, I will leave you with some ideas to finish off your opponent, but it ultimately comes down to the situation and personal preference.

The goal of this strategy is to abuse the mobility, range and shield regeneration of very early stalkers, void rays, and warp gates to make it very difficult for the terran to attack you or expand, allowing you to expand, tech, or just win outright. It also allows for a fluid, easy transition into zealot/templar for the mid game, and tends to force the T to get an anti-armoured and anti-air mid-game composition, making your zealot/templar even stronger.

The key is to put constant, non-stop pressure on the T starting at about the 4:30 point, preserving your troops while whittling away theirs, and only engaging in a full battle on your own terms.

To do this, get stalkers quickly and send them to your opponent's ramp as soon as they spawn. You should aim to get them there early enough that only a couple of terran units are made. Push up their ramp and take pot shots at the marines/marauder, or their wall if they pull their troops back, and then just fall back and let your shields recharge; rinse and repeat. The key is to try to hurt as many soldiers as possible (since medivacs are a long way off), and to damage the wall so you can snipe scvs if he goes to repair. Keep reinforcing with more stalkers and do more harass.

Avoid making more than 2 gateways until you have a stargate. It is likely that your opponent's build will skew towards marauders and/or tanks since stalkers are so good against marines and hellions. When resources permit it, tech to void ray (I usually begin my stargate when I have between 2 and 4 stalkers). The void rays tend to start popping when I have approximately 6 stalkers and 3 warp gates. The void rays provide an alternative method of harass, can give vision of the ramp to your stalkers, and can shoot the SD's and barracks that are walling the ramp if they have chosen to put them there. It is so easy to micro your void rays when there are a bunch of stalkers underneath that outrange marines, or if the terran has their own wall blocking their way.

If you continue this harass, and avoid committing to close battles with the terran army, you should gain enough of an advantage to be able to safely expand or tech. Not only that, but you shouldn't lose many units if your micro is crisp, ideally leaving you with an economic and army advantage.