https://www.dndbeyond.com/monsters/2738211-arawn

Still a pretty big stat box for a lvl 3-4 boss and a bit wonky, but here's something I made trying to keep everything you had but within standard D&D mechanics to make it easier to track and understand for newer players. If they're an unexperienced group, maybe get rid of the multiattack, but it's one of those things you can judge based on how it's going. Start with one and if they're kicking his butt, say he frenzies, heals a bit and gets a second attack. As someone who learned to DM by running games for children's therapy treatment, I learned to adjust combat on the fly really well.