A Bandit Captain and their cohort always make for a good boss at lower levels. It's believable and easy to tie into a previous encounter; plus, their difficulty can be adjusted easily. Depending on your party, you can add or subtract health, number of bandits, magic items, or even alter...
Bandit Captain Medium humanoid (any race), any non-lawful alignment Armor Class 15 (studded leather) Hit Points 65 (10d8 + 20) Speed 30 ft. STR 15(+2) DEX 16(+3) CON 14(+2) INT 14(+2) WIS 11(+0) CHA 14(+2) Saving Throws Str +4, Dex +5, Wis +2 Skills Athletics +4...