The reveal and fight between the two must have been very dramatic and intense lol.
Also I think that is a different scar compared to the initial first scar we saw she got from Hana in the previous...It is a diagonal scar...The scar we saw in previous chapter was straight line it seems/Vertical type (Of course it could be the same scar and cut butt author just forgot the angle perhaps???)....Plus at the end (Pwn/REKT Juuna/Evil general laying in a pool of blood...Or is that her hair...blood or hair?) So yeah...Definitely more than one single cut from Hana's sword [That scar (Diagonal type probably was the most intense injury that couldn't be fully healed even with magic apparently???)]
And this is why kids...You do not let your enemies grind EXP and level up to their ultimate, uber, super, mega, hyper, amazing, extreme (Add about +6-7 more adjectives) final transformation mode!!!! Good news for Hana though that Juuna took too long for the eventual reveal and battle (Probably got caught in the feelings and couldn't bring herself to put her 100% intent to hurt and kill Hana either lol...)
Yes I am just joking around with all the adjectives and it was just one single powerful/level up form (Wing heart gem instead of normal base gem) but that is one hell of a power up.
Juuna got rekt/pwn...Hana was feeling h0rny but now she feeling guilty/remorse...RIP Hana XD
Don't worry Hana, it not that uncommon to get into intense battles with your significant magical partner at the beginning...Or at the end game(Due to misunderstandings) (Looks at Nanoha and Fate...Then at Final form Madoka and Final form Homura.)