決める +としよう = plan verb + volitional


2人に一緒にかっこよく決めるとしようや

The characters are at the end of a challenge and they are about to face the
last room, when one of them says this. After that, they do an athletic c trick
to pass the challenge.

The way he's saying it looks like he's saying it "Let's carry it out with
style", however 決めると is in its plan form?

