Purchasing card with Exile keyword going into the final scenario is already a good tactic. But for this card it is even better. Penultimate / final scenario has a trend : designers love to add Ancient Evils set around here to make remaining time unpredictable, and round loss is likely with heavy consequence. Therefore A Test of Will (1) is like purchasing rounds of play with little amount of XP.
It has "lite" version of team-coverage feature compared to Ward of Protection (2) (requires same location), but that already fix more than half of the biggest problem of unupgraded Ward of Protection (0) vs. its highest reward target Ancient Evils : It must land on the Mystic otherwise there is no way for the recipient to resist.
You can go in with some strategy, like grouping up tighter a few rounds in until 1 or 2 out of 3 Ancient Evils passed by. It is relative to when you actually draw this card vs. how many evils are left at that point. But you can also try mulligan to get this then you get to defend against 3 evils for sure.
I just came back from a certain campaign with tight location layout on finale. While we won, amount of rounds left is scary low. Factored in how lucky we were that Ancient Evils often land on the party's Mystic when he has the Lv. 0 ward on hand (he only has enough XP to upgrade 1 copy to Lv. 2), if luck was not on our side we would have lose and I regret I didn't think about getting this card with my leftover XP. A team with Survivor and Mystic will be at huge advantage on keeping the evils away.