Hey man. I would choose Hazama for an easy 9/10 , Arakune for an easy 9/10 , tsubaki for an easy 8/10 , Lamda for an easy 7-8/10. Also the 2 DLC characters are very easy- Makoto 8/10 and do valkenhayn 6/10 ( dont do 7 as hes achievs are currently glitched ). If you want any more help just let me know. The fact that you did 10 with jin and Ragna makes me think you will have no problem getting superior state of mind.
Saw that you completed Superior state of mind recently, do you remeber at around what percent it unlocked for you? And who do you suggest you try to complete challenges for? I already got 10/10 for Tager,Ragna, and Jin but I don't know who I should really try to work on next.