An artificer takes Craft Contingent Spell, and either Leadership or some other means of gaining three assistants (I'm not sure if constructs would work; I'd have to double-check all of the relevant spells). He crafts four Contingent Spells of Delay Death, four of Share Pain, one each of Glory of the Martyr, Masochism, and Beastland Ferocity. The triggers for each of these can just be him saying a particular word. In addition, he has a nonmagical thinaun dagger, and three more contingent spells: A Shatter, triggered to go off targeting the object in his right hand if he's been dead for more than ten rounds, a Revenance, triggered to go off targeting himself if the object in his right hand is destroyed, and a Revivify, triggered to go off if he dies due to the duration of Revenance expires (these last two could be replaced by True Resurrection, but this is cheaper).

You may recognize what I'm doing here-- It's basically just the omniscificier, re-done to not be reliant on questionable things like custom items and drown healing (which forces him to be higher level, but that's OK). In addition, he has also crafted, oh, let's say a half-dozen scrolls of Wish.

Here's what I expect will happen. He grabs ahold of his dagger, triggers his omniscience spells, and jumps off a cliff. He takes infinite damage, which attracts the attention of The Entity, but also gives him full knowledge of it. 1d4+1 rounds later, if it still exists, The Entity first strips his class features (which he no longer needs, just his already-active spells, his existing items, and his skill ranks), then attempts to kill him via nigh-infinite damage, which is redundant as he's already taken infinite damage and his Delay Death is still active. The Entity then uses its just-die power, which (presumably) works, but the Soul Bind does not, as his soul is instead sucked into his dagger. Ten rounds after that, the dagger is shattered, his soul is released, and he's brought back to life via Revenance and eventually Revivify, with no level loss.

Meanwhile, once he attains his omniscience, he's going to use his scrolls of Wish to attempt to take down the creature, in whatever way seems most appropriate once he knows all about it. This can presumably be done within 1d4+1 rounds, since the wizard's Foresight advised him that the way to avoid death was to kill the thing first.