How to use octavious or how it's called???

*reads through manual*
Sorry, never heard of it.

octavious
You mean the green sea monster that grabs your opponent from below? That somewhat looks like an octopus?

It's used like any other weapon:
Just press the fire key or use the weapon switch key if it's not selected.

If you are the victim of the sea monster, you need to quickly press the fire button multiple times until you get free of its grip.

I know how to send octopus to enemy, but, what I should press when the enemy is captured?

I know how to send octopus to enemy, but, what I should press when the enemy is captured?
Nothing. Let the monster do its job.
Your enemy has to worry. :)