My conversion from 3e/Pathfinder to 5e D&D system:

AC: Halve AC over 10.
Hit Points: Increase by 50%.
Attributes: Halve attributes over 20 (monster) or cap at 20 (NPC)
Damage: Increase by 50%.
Attack Bonus (weapon or spell) & Spell Save DC: Calculate based on CR-derived Proficiency + Attribute bonus
Good Saves: If it's a Good (2+1/2 level) save in 3e, the monster has Proficiency in it in 5e.
Will > WIS and possibly INT or CHA.  Ref > DEX. Fort > CON and possibly STR.

This is easy enough to do that I can convert during play. I rarely worry about the exact number of spell slots an npc caster has, they will be dead in 2-3 rounds anyway...

Read more: http://www.enworld.org/forum/showthread.php?545337-Is-there-a-conversion-of-the-quot-Caverns-of-Thracia-quot-to-5e#ixzz4fukKzlQK