Spell Shield is now available (but stops you from casting when it ends, for x seconds).
E-denial build
Cast Blessed Aura + Quickness of Serpents + Spellbreaker (also should have a weapon/off-hand item that lengthens echantment duration).
Interupt build (must have good timing)
Skill denial - signet of humilty (mesmer - disables foe's elite skill for x seconds).
High Health, and have a monk
low max health = low loss = less healing to him & easier to heal for your monk.
using GB against high health bosses is the fastest way to kill them...same for you
You can't lose more than half your current health due to a single use of Grenth's Balance. That's a fairly effective 'counter', in my opinion, though in some occasions, comboed with other spells, it's still enough to kill you.
(Though, since one of the primary skill combos with this is Phantom Pain->Shatter Delusions, you do get some warning of what's to come usually.)
And of course, all catch-all spell counters (Obsidian Flesh/Spell Breaker/Shadow Form/Spell Shield) are effective deterrents to being targeted with it, though I wouldn't rely on them, as most professions can not really keep them up, or even have any such skills.
Your best bet is to watch for characters that can kill you with Grenth's Balance, and watch their health. With effective spiking, you can snipe a target that has reduced its health to take a huge chunk of yours, and seeing a character like a Me/N with very low health tends to be a strong indication you're going to be spiked in a moment.
I would have thought a heap of degen would be enough to counter it , stacked the cooldown for grenths and taking massive degen from hexes means your likely to die before getting your GB off. E denial probably the quickest fix though.