I like Dan's suggestion of "Shall we have a go?" But I'd like to add that "bash" can mean "party," so it could also be something like "Let's have a party, then!" (This is coming from someone who grew up in Canada, so the bash = party obviously could be just a regional thing.)

There's not much I can add beyond what Dan's already said, but I just wanted to add that I've also never heard anyone say this. Though, I'm not actually sure what message you're trying to convey. If I had to guess, I'd suggest you were challenging someone to a fight?

That could mean many things depending on context. It can also be taken the wrong way, like an invitation to a fight or be intimate with someone. For this reason, people may make a joke after you say it, or may be offended. So I recommend never using this phrase ever. Slightly simpler & cleaner alternatives, that may work (depending on context) include: Shall we have a go, then? Well then, shall we give it a go? Shall we try, then? Okay then, shall we do it? Are we going to do it, then? Alright then, let's give it a try? Let's give it a whirl, then? Let's have a go, then?