If you haven't yet, visit the reddit sidebar:
https://www.reddit.com/r/grandorder/
And check out beastlair kyte's posts, in particular:
http://blogs.nrvnqsr.com/blog.php/1056-Mining-for-Bits
The posts on ascensions,
And servant profile at:
http://forums.nrvnqsr.com/showthread...eplay-Profiles
Or if you prefer use the Japanese wiki:
http://www9.atwiki.jp/f_go/pages/1.html
Sabers are primarily damage dealers, along with lancers and berserkers. They work well with either one of them. Like what GDB said, try not to place them in teams that are quick focused, especially assassins, and to a degree riders and archers. They can supply damage options to arts team as well.
I would also suggest leveling servants of different classes evenly. But when raising them, you may want to plan long term, since mats (especially like hearts, scales, and those that can't be acquired in-game through farming yet) are more efficiently used with gold servants.