Well...the fact that you'd go Force off of an A-Ranked spell instantly was due to the AMOUNT of element used within an A-Ranked spell...for instance an A-Ranked amount of water is a shit ton. Even if the spell is just a direct water blast, there is still an A-Ranked amount of Magic within the magic. Which is a lot.
S-Ranked and above should definitely be instant Force. If you're dumb enough to use a spell that powerful on a Slayer who can eat that element; then you deserve to get Force Mode up your bum LOL.
Perhaps 2 A-Ranked spells of your element at the most should be enough to use Dragon Force. 4 B-Ranked spells of your element. 6 C-Ranked, 8 D-Ranked.
^
This is fair in my opinion. Making it actually attainable. Cool discussion though, that's all I have to add to it.
S-Ranked and above should definitely be instant Force. If you're dumb enough to use a spell that powerful on a Slayer who can eat that element; then you deserve to get Force Mode up your bum LOL.
Perhaps 2 A-Ranked spells of your element at the most should be enough to use Dragon Force. 4 B-Ranked spells of your element. 6 C-Ranked, 8 D-Ranked.
^
This is fair in my opinion. Making it actually attainable. Cool discussion though, that's all I have to add to it.