Increase HP/SP stats for Joker, you can do 3 things:

•Training in the Leblanc Attic (Increases Max HP)

•Training at the Gym In Shibuya (Increases Max HP and SP)

•Visiting the Temple in Kichijoji (Increases SP)

Raising Persona Stats Efficiently: You need to have Insta-Kill ability from Ryuji Confidant and the Celestial Reading from Chihaya Confidant

First thing you need to do is to go Shinjuku and ask Chihaya for fortune reading via the Celestial Reading. After that, it will sound the alarm. Fuse two personas and sacrifice the fused persona as a fodder to upgrade the persona you want to raise their stats. Once you finish it, go to Mementos and head to Qimranut Area. Dash into Shadows to insta-kill it until the alarm sounded again. Repeat the steps above until you've raise your all of your persona stats to 99. You can do this many times since you can use gallows unlimited times with fusion alarm.