An easy way to get almost free XP is to go after Drug Dealers since they are always guilty.
You can easily level up for the first time quickly just by arresting 10 NPCs, so it's good to try using this for the first early floors.
You dont lose the XP bonus if the Handcuffed NPC dies so you don't have to worry about accidentally hitting them.
Arrest (Innocent)[]
Gives you +10 XP when you use Handcuffs on an innocent NPC but makes you lose 80 XP if you have The Law, this can be reduced to 40 XP if you have Crooked.