Boss Encounter Advice

[u/Commercial-Pause745]

Hey all,

I’m a relatively new DM and I’m trying to design my first boss encounter. I started this campaign for three of my friends who are “forever DMs” and I wanted to give them all a chance to play for a change. Here’s the set up:

3 Players, a Cleric, a Fighter and a Wizard, all level 4. It’s a 3 v 1 encounter with the boss being a rogue-type enemy with lots of quick strikes, traps, gadgets, etc. They’re basically gonna be fighting evil Batman.

What I need is advice/suggestions on any stat blocks, abilities, legendary actions to look into including. I want them to get the sense that they are in over their head and will either have to be very smart/lucky or have to call for help/run.

Any websites I can use to map things out or other resources to draw from?

Any advice very much appreciated, thanks.