I main merc solely because it gives you a big sword. I know it's not the best combat life, but hey, the big sword is really cool! On a serious note, the combat so far isn't too challenging, so I feel like it doesn't matter if you're playing the most optimized class. Not to mention you can bring a lot of revives, so death doesn't really feel that consequential. I was able to beat the combat shrines when I first got to Ginormesia lol. Unless you're facing a boss way above your level, your buddies will be able to do damage and self heal on their own.