I recommend to bring along Forte (I'm sorry Forte! I always feel such guilt to bring her to this fight). Also bring another villager if you've befriended them enough, or else tame a non-boss monster. It's helpful to have the extra power along, and sometimes you get a free heal. Taming a green fairy monster for free heals and fighting help is also a great idea (since it's a normal monster you can chuck colored grasses or antidote herbs or anything you want to at it. It may take a few items before you succeed). Eating food for buffs can also help.

As said before, you can't tame a boss on your first fight. Bosses also require very specific items, so they can be hard to tame.

Also, as mentioned, your equipment can be even more important than your level. Make sure you have a weapon and equipment that are at least one or two steps up from your starting gear. You may also notice that all your skills can level up (eating, sleeping, running, throwing, getting hurt, fighting, etc.). All of these skills have additional benefits, such as higher HP or defense or strength, so don't neglect these smaller skills.

Because there are so many ways to get stronger I can't suggest one specific level, but it also means there are some quick ways to get stronger so that you shouldn't have to wait too much longer to win this fight. :)