Help with running Silver of the Sea

I've recently started to DMing some of the Vaesen scenarios, more specifically the Silver of the Sea. >!​!<Until now, everything's been going okay, but what worries me is that the ending confrontation might end up being a piece of cake for them since one of my players decided to buy a cannon. The Kraken might give them a scare but it would be extremaly anticlimatic if he just oneshots the Mermaid.>!​!<
So basically, how should I deal with that?
Just to point out, I'm really new to being the DM in any RPG whatsover so any tips on how to run this scenario could be useful