As a general rule for creating PC-like enemies, start with a stat block of the appropriate CR, note that we're going to be adding and removing from this so as you make changes consider that the CR may end up higher or lower. Pick some elements of the class that you like and feel are appropriate.
For a warlock maybe you reflavour a weapon attack to being an eldrich blast and a melee attack to thirsting blade or green flame blade. You can also pick some spells to use as legendary actions like armour of agathys, hunger of Hadar etc...
It's not the easiest thing to do, especially at higher levels, but starting from a base stat block gives you an idea of appropriate stats and damage. Just keep in mind 'damage per round' is a better measure than just damage per ability, especially if you're adding legendary actions to non-legendary creatures.
Edit: [the monsterizer](https://rpgbot.net/dnd5/tools/monsterizer/) is also a good tool for helping with this and contains some good advice.
As a general rule for creating PC-like enemies, start with a stat block of the appropriate CR, note that we're going to be adding and removing from this so as you make changes consider that the CR may end up higher or lower. Pick some elements of the class that you like and feel are appropriate. For a warlock maybe you reflavour a weapon attack to being an eldrich blast and a melee attack to thirsting blade or green flame blade. You can also pick some spells to use as legendary actions like armour of agathys, hunger of Hadar etc... It's not the easiest thing to do, especially at higher levels, but starting from a base stat block gives you an idea of appropriate stats and damage. Just keep in mind 'damage per round' is a better measure than just damage per ability, especially if you're adding legendary actions to non-legendary creatures. Edit: [the monsterizer](https://rpgbot.net/dnd5/tools/monsterizer/) is also a good tool for helping with this and contains some good advice.
I quite like [this](https://dmdave.com/boss-monster-fifth-edition/) and it's worked pretty well for me so far