Actually, Imu also tried to use the attack on Luffy, but he dealt with it twice. The first time was when Loki was defeated, and Luffy in his base form managed to handle the attack that impaled Loki, and the second time was in this chapter, at the last second/point-blank.
Luffy came out better in both cases, especially at close range (point-blank), against an attack that hit Loki even from a very great distance.
If it were Loki, he would already have been defeated — which by default means the attack that beat Loki isn't enough for Luffy, and that makes Imu raise the level of the fight to one completely different from what Loki faced.