Luffy punching Imu in itself is not a Big Deal, it's not like he got one shoted or anything - it's this typical ''hype'' MC entrance bs.
But i wanna see how it happens, i am afraid Oda just ignored everything we saw about Imu, just to give Luffy this moment. Wouldn't be the first time, Plot > Logic...
But now it seems like a 2 vs 1 and if they can't overwhelm him, then Imu played with Loki. Which was obvious, considering how he blocked and dodged everything Loki threw at him, speed blitzed and stabbed him while casualy talking. This ''even'' fight right now is just to stall the plot.