Buffer

Well, you can put a Rally support skill on your heroes, so it doesn't need to be through C skill only. Some heroes can buff blade tomes after attacking if they stand next to them - Sanaki, Ninian, Fae and some others I guess, but I really can't remember now.

I often use Sharena in this role and gave her Drag Back to pull targets into nuke range. She's not quite as good of a buffbot as the Sacred Stones duo but she's available to everyone and is regularly a bonus unit.

I use Ephraim with Hone Spd. He can buff my Tharja's offensive stats while my other two units handle her defensive stats.

You can use other characters.

If u use 1 bladetome unit then just give 1 unit hone attack, hone res 1 sacred seal (if u have it) and rally speed and u have Eirika/Ephraim buffs but missing out on rally defence. Besides give only 1 other unit Fortify Def 3 and your all good to go. The main reasons why the twins are the best buffers is that only 1 unit is needed as opposed to 2 units to give the same amount of buffs.