Since int is her cheapest stat to raise, and her original skills are based off her magic stat, it makes since to develop her as a mage. The current board axiom seems to be: high vit, high agi, high attack stat. So it sounds like you are on track.
"That was pretty interesting. But dropping a warship on me is cheating... Take it back!" - Xenogears
If your going to only put Int/Atk on sythe user then you should just use a mage or a fighter. While Carla is a better magic user then a fighter, she will never use magic better then a well trained mage. Sythe users are ment to be hybrids and while they can get away with focusing on only one stat(i.e atk for Hiro and int for Carla) they lose some of their effectiveness.
When I grow up I want to be Dracula or Snoopy or maybe even a crayon.
uhm well not really, Carla is much better for int and if she casts high ap-spells she won't have ap left for physical Attacks. And 90 int and 12 str with only magic is better than 45 int and 45 str with both. (you won't get more at avg, cause int is much cheaper than str for her) and the best Scythe (G.O.H) doesn't even have a slot for physical attack. Then all her Originals are Magic-based, too. So Carla is a pure Mage imo and damage-wise she can do the same than Snow. (Noel can do more in critical, but thats because of the potential only)
As for how I build her: Well I didn't use her in the beginning, only later after my other chars where lv 90+, so I leveled her on my own chars and I raised almost only vit until she got 92, and then agi/int to about the same. So now at lv 100 she has 43000 hp(86000 with hp up+) and she still didn't get much lvs after 100, maybe 2 or 3.
I looked through them again and well her first has 2 physical hits and 3 magical, second is to boost Magic-attack (with this she can outdamage Snow) 3rd and 4th are only magic-based. As for self-combo, she has Ice, so with a Lightning Weaponslot (like G.O.H) she can do Turbulence which is quite good (huge range, lots of hits for 29 ap)