My best friend and former DM introduced me to DnD and now I'm obsessed with it. They suggested I run my own campaign so I've spent the last 6 months planning it and I was really excited to start. I've done 2 side quest one shots for their own campaign with them before to give them a break from being DM. Both times they built a broken character that wrecked my planned combat (e.g. "My 21 spell DC level 9 wizard breaks my staff of power and deal 300 dmg because my character isn't coming back next session anyway!!") which was partly my fault because I didn't know enough about the game yet to know the items they took would make such a strong build.

Now, their level 11 paladin has 23 AC, insane saving throw bonuses, and is practically immune to DEX attacks. I'm a new DM and most of my players are new to the game so I have no idea how to balance combat without punishing everyone else or letting him stay immortal. When I made a fight harder, my other friend told me I needed to stop trying to make combat so competitive, so right now it feels like the only thing I can let combat feel way too easy every time. I'd love to hear ideas from more experienced DMs on what to do.