Support Grinding

They have to stay within 2 spaces of each other during combat. Unit supports shouldn't be nearly as hard as summoner support otherwise (took like a dozen and a half battles for Lyn to get support A with me)

I got one upgrade while farming SP in the arena but, in general, Tempest Trial seems to be doing the trick for me.

Tempest Trials when it's out, Arena Assault for free, and Normal Chain Challenges for easiest farming.

You can basically just go SP farm for them, making sure they're in support range of one another and then just clear the maps.

Here's a video for some good info on the Support System so you can make your own judgement:

https://m.youtube.com/watch?v=w2nKDLOMNvA

If I remember correctly you need to fight units above lvl 35