Leon +Hp -atk

you got -atk >///< I mean personally I rarely ever consider anyone -atk unless they have blade potential or are not going to be used for attacking

Personally, I'd just give Slaying Bow to another archer who could use it better, or send him home for feathers. If he was neutral/+atk I would say yes.

I've got this same Leon. I don't think he's worth investing but I've been using him for arena since he's my only 5* bonus unit for this week and last week. I leveled him up to 40, but I haven't put any inheritance skills on him. I've been thinking about giving his Slaying Bow+ to a faster unit like Setsuna or Rebecca.

At neutral stats if you compare to other archers he's got the 2nd highest DEF, the 2nd highest ATK, 2nd lowest RES, and the 3rd slowest SPD. His -ATK bane puts him in the middle of the pack.

But if you really wanted to keep him like if you really like the color purple then maybe his best chance is with Death Blow 3 and either Bowbreaker, Blue Tomebreaker, or Green Tomebreaker, and the ATK+1 seal. This way he can KO somebody on his first turn with a breaker skill then his Ignis should be ready to go on his next attack.

Not for competitive use. If you like him you could use him in the Tempest to gather hero merit but stay clear of the arena and similar. Also, if you do like him you could always save him to merge with a better IV Leon if you get lucky down the road. Personally I'd probably use him for skill fodder but wait until after I had farmed him for hero merit.