Yami DF is the only power to negate Imu's ability, but the user is not immune to Imu's hax. That's why Teach setup his crew to be infiltration style. His plan is to sneak up on Imu and touch him with his black vortex, kinda like the way he sneak up on another hax user Boa Hancock. He knew what those hax powers are capable of, so he smart enough to stay under the shadow and let Nika becomes the bait for Imu until the right condition to act.
Nika DF power is immune to Imu hax, and maybe can defeat Imu too, not 100% sure for now.
What's better to defend against other hax, having another hax that can protect them against all kind of hax.