Raid enrage mechanic

The countdown goes down on each enemy/boss round. That's why top raid teams are often based on Speed Down and TM reduction.

It also depends which raid you are referring too, rancor or tank? Rancor is a tmr based squad, tank it depends on which phase,

A couple things to note about enrage. The countdown timer is reduced by one at the beginning of the boss' round, which means that when you see the countdown timer at 1 Ior 2 during a topple in P2 AAT), on the next boss turn, it will enrage and do massive damage. So if you want to try to Escape some characters, plan accordingly.
Also, the countdown in P4 Rancor is bugged. The visual counter will decrease by 2 each boss' turn, but the internal counter will correctly account for the number of boss' turns, so you might have to keep mental track of what the counter should be once the visual counter runs out.