In PvP, how should I respond to the tentacle proc from Gurthalak?

Solution 1:

The mind flay ability that the Tentacle of the Old Ones casts has a 40 yard range, although like most channelled abilities it only needs to be within that range at the start of the cast, and requires quite a bit more range to break the channel. I would expect that constantly attempting to outrange the tentacle would not be an ideal solution.

The tentacles can also be targeted and have 15k health and can be killed. They are classed as 'guardians' so you could enable health bars for enemy guardians to make them more visible. This means that you should also be able to use a /target macro specifically for targeting tentacles as a counter for this ability.