Rex meta any real counter

As the previous two posters have said, Wiggs. They can one shot Rex easily, and if you put your best speed secondaries on Biggs he will shoot first. Rex's leader skill will endure, but he'll not use his initial TM boost. 7% TM per crit still matters, but when those crits are doing 20k+ damage they don't add up quickly enough.

I've had a fully maxed Rex for months and I don't run him as leader, I run Lando and get first strike every time.