The only reason the ability isn't popping immediately is because you are still under the effects of the cast you just finished. The tactical nature of the game requires a little more anticipation when casting things like Benediction. You had 2.5s of GCD to realize your tank was going to die. You should have either A. interrupted whatever you were casting to use Benediction or B. Find a tank that can survive more than 2.5 on his own.

People keep mistaking this battle design for action when it should be categorized as tactical.

It's like saying "I didn't know I was gonna run out of MP by spamming Medica II".