Mothraya

Mothraya is a giant, robotic moth as the first boss in the Cossack Castle.

Mega Man 4
Mothraya will fly around the battle to kill Mega Man. He must use Ring Boomerang at its red button, also Drill Bomb and Dust Crusher are good options. Also, he must slide under it to avoid the spike that destroys a part of the floor and must be careful, as he dies when he falls into the pit.

Simply, Mega Man must not try to slide where it'll fire its spike as that can attack him and he dies.

In normal mode, the spike will damage him a little.

In hard mode, the spike will damage him badly.

Trivia
Mothraya is a nod to the Japanese movie giant moth monster, Mothra.