Yes you can kill every MHW boss again and again, all you need to do is to talk to Alma.

Guide

1. Find Alma and talk with her.

• You can always find her in base camps next to your tent.

2. In Quest Menu tab click on the Post/Join Quest.

3. Select Optional Quests and choose the monster you want to fight.

4. Click on the Accept & Depart to start the quest.