I was in a FFA with 2 other Protoss and a Terran... towards the end there were a ton of static Photon Cannons guarding bases. I was thinking Immortals with a few Sentries for Guardian Shield would be best... but I wonder if there are any other tips or unit ideas for cheaply breaking up a giant cannon farm to attack an enemy Protoss' base in PvP
Colossi with Extended Thermal Lance have a range of 9, while Photon Cannons only have a range of 7. You can snipe off the defenses without taking any damage.
The slightly lower tech option is immortals, who do their full 50 damage to buildings while only taking half damage to their shields (20 maxed at 10).
Expanding a lot and building up a large, upgraded army is really what it comes down to..... making cannons early or even late can but a Protoss pretty far behind.
If you do not have any money and only a squad of stalker, that should work too with blink micro. But usually it is not worth the time investment.