Like it was stated in one of the posts, Twin's problem is weave and power weave low cooldown which allows them to be almost spammable with some timing. Easiest way to fix this is to increase cooldown to 3-5 seconds each. This still allows the use of the skill in PvE because I'm pretty sure you do not need 100% block in PvE as much even if your skill level isn't so high as all bosses have so much delay between their attacks. In PvP, this makes the user think twice before using the skill. Having them shared cooldown like some suggested makes Power weave pretty much an upgraded version of weave, why bother using the other if one is superior in every way in any situation.
For Summoner's, a major problem for them is their reflection skill on summons which renders 50% of the classes helpless against them. Pretty sure this skill isn't needed at all in PvE. As the summons are always around the Summoner, classes that take full reflection damage cannot attack the Summoner without taking damage themselves which the reflected damage is always higher as summons take PvE damage while the Summoner takes PvP damage.
Suggest a lower reflection rate such as 10-20% reflection rate at 50% of the damage taken? Or even set it like Meteor Earrings where it's 20% reflect rate for a fixed amount of damage.
As for their x-spam's, what makes their x-spams along with other x-spam classes so op is the chain combo flinching which almost allows for a lock with high enough atkspd. I believe only Summoner's class is able to hold opponents still due to x-spam flinching. Invokers and Destroyers cannot achieve the same flinch lock to the same degree. 50% reduction in flinching is a suggestion or even removing the flinch all together. Without flinch, even Summoner's OP x-spam isn't too much of a threat........
For Invoker's magnet, an increase in cooldown would be the easiest fix, although 3 seconds sounds a bit short. I think the cooldown is not what makes magnet so overpowered but the interval in which it pulls the opponents.
How about keeping the cooldown time but increasing the interval in which it pulls the target? In PvE aspect, once the mobs are pulled, they are stuck there so it doesn't matter. In PvP aspect, usually a barb or some sort of stun/freeze/crowd-control skill is coming after the first pull so increasing the pull interval will actually allow opponents a chance to escape and does not affect the invoker's combo/kill-ability too much. Magnet actually has a pretty long animation cast time which makes it avoidable unless the opponent is caught off guard or is unskilled in dealing with magnet.
Suggest an increase in 0.5 seconds to 1 second increase in the pull interval. This still allows the skill to do what it's suppose to do but allows that slight chance for the opponent to escape.