I have three gladiators at level 60, which is the maximum level btw - but i can't beat the 20 roman contender battles for my last missing achievment. My gladiator's dexterity lies in between 84 and 99 but less than 50% of their attacks hit the target. Their speed is even higher (91-110) but about half of the enemy gladiators attack before my gladiators start their first turn. That's quite unfair imo.
The speed attribute seems to be bugged. Although all of my gladiators have the same stats and the same equipment (2 times 40 speed), two of my gladiators (+ all of the enemies) attack twice before my third gladiator can start his first attack.