Casts "Overcharged Arcane Explosion" for 6k arcane damage. While not interruptable, it has a 4 second cast, allowing you to run out to avoid it. Emotes "Crystalcore Sentinel's hand begins to glow with Arcane energy." when casting starts. Since the Sentinels come in pair of two, it is also required to separate them so that if both of them use their Explosion at the same time, the DPSers don't suffer too much.
Casts Overcharge, 1.5 second cast, 14-15k Arcane damage to the main aggro target. Not interruptable, but it is reflectable.