Help me with my min/maxed Paladin / Warlock player!

Quote from SethZeal >>

The other players certainty haven't complained and they all get along and have wonderful cohesion. There's been no issues that I'm sensing as a DM with high empathy and knowing my players well.

It's definitely a problem for their enjoyment or anything. No one feels left out or outshined. They strictly just dont feel challenged. Unfortunately it's this particular character that's making it hard for me to do that without risking TPKs.

The other issue that's making it hard is this isn't a homebrew campaign. This is a narrative weaving of about 4-5 published adventures to create a single campaign epic that culminated in Rise of Tiamat. They're current level 9 and one more session away from the level 10 power spike (though nothing quite like the level 11 spike lol) and beginning of Tier 3 play. I'm trying to figure out how to tune the remaining combat encounters so that they feel at least moderately challenged without having to scrap them all entirely just so I can build my own in spite of (I use that term loosely) a single player character.

I just dont have enough experience building my own combats to know where to begin other than light tweaks. Additionally enemies of the types already there. More HP. So on and so forth. Any input would be appreciated though!

As for healing, they have the Paladin of course, and a Druid, and a somewhat healthy collection of potions.

 Give the other players things to do that align more with their strengths.  Look at mobility, flying opponents, look at skills that the Pally doesn't have.  Use Heat Metal, use obstacles that cannot be bashed.  If he is a hammer, don't let the encounters all be nails.  

As another solution, have you spoken with them?  In 3.5 edition I had an Ultimate Magus, which was a crazy Prestige Class (doesn't really matter).

What mattered is that I had enough spell power that I never ran out of spells, and just mowed down the opposition.  The DM spoke with me and we decided to change that character to a standard wizard, (we RP'ed it in dramatic faction with an unknown entity ripping my innate sorcerer abilities away) and off we went, problem solved.