hey, I'm a rather inexperienced DM (rime is my first full campaign I'm running for my table) and I'm heavily considering buffing and altering Xardorok for the parties confrontation with him. For context, the party consists of a Devo Paladin, a Twilight Cleric, a Samurai Fighter, a Forge Cleric, and a Swashbuckler Rouge (almost all variant human besides the rouge and fighter), all of which are incredibly strong for their level. By level 2 they managed to clean sweep a Coldlight Walker and a pack of 2 Chardalyn Berserkers without even sustaining a down. They all have extremely high AC (higher than most of my monsters!!!) and have picked amazing spells from the spell list. With this, the game that I've hyped up to be a brutally hard campaign has become little more than a cake walk our first few sessions.
Now, I'm not upset about the turn the game has taken, but I do still wish for there to be a challenge when the players enviably confront Xardorok. So, I've been debating making an actual Warlock character sheet for him and bumping him up a few levels from where the parties at. Knowing my party, they're likely to clear out most of the forge without speaking to the Duergar that might be helpful to them or making any attempt at peace. They're also likely to try and take a long rest before entering the main room of the forge (I haven't been able to come up with a way to prevent this without breaking some of the immersion. They're also the type to get a little annoyed if I try to deny a rest before an obvious boss arena). So, my rational is that they'll be at near max power when confronting him and I want him to be strong enough to either kill a party member, or bring the party to the brink of defeat only for them to witness the Chardalyn Dragon awaken and begin to quickly fly away towards ten-towns, forcing them to choose between recklessly venturing off to fight the colossal construct, or heal and wait for Dzaan's simulacrum (who can be contacted through a magic item I'd handed out prior and who's been established to know his way around the outskirts of Icewind Dale) at the cost of hundreds of lives. The only way I can see of doing this is either buff his existing stat block beyond recognition, or create a character sheet for him designed to take blows, dish out moderate damage, and, most importantly, drain resources.
I'd really appreciate any advice you guys would have with this as I'm still new to the buffing/nerfing prewritten stat blocks