You just need to know what Frey abilities are by heart. Or look it up from the encyclopedia:
3x damage + self-burst+15
Self-crit+ (+?%) + self-mode gauge bonus (+?%) 5t [7]
Rage damage+++ 1t [15]
... oh, it looks like I haven't updated it with Frey buffs. I'm pretty sure abi2 has more buffs than that now.
That abi4 -> abi3, abi1 is going to be a monster in the right conditions. Turning your entire team into a Rage punisher is NO JOKE.