Summoners pets scale of T-Atk (Tech) so you would build your units around that along with Tech attack cookies as well. The Class CANNOT use JA's (Just Attack or Perfect Attack) so anything giving a multiplier for those will not be reflected on your pets damage (as your attacks are considered Normal or Non-Perfect). Since we just had the CBT, I won't get straight into the any of Candy Box builds as I don't know what the NA names will be translated to so I'd like to not confuse people with the JP names if possible.
New Players are recommended to pick up Su/Fi until both classes reach (or pass) Lv 75/75 to be able to unlock the Successor (or Advanced) classes to be able to set Phantom as a Sub class.
For the time being, here is a a skill tree build to use, naturally you're allowed to move things where you see fit if you're not content with the build but I tend to post builds that are mainly set for the current Meta: https://tinyurl.com/whmth3m
I'll keep this post updated as the game changes as much as I can as well so people don't have to dig through the thread looking for info