You mean which BT on Chuunin Shika right, not Kushina? For Shika, definitely the first one you want to get is his Hidden Jutsu Strength passive skill to +2. This is what buffs ninjas like Kushina and Fire main and is the main reason he's used on those teams. It's good for both offense and defense in the first two rounds.

After that...I'm not really sure. His second chase, Shadow Mimicry, can be really nice at +1 where it cancels shields and buffs. However, if your team doesn't always trigger this chase from him, you can go for his standard attack to +2 instead. That 15% increase in damage might not seem like a lot but I've seen his standard do more damage than you'd expect, especially in the first 2 rounds while buffed by his Hidden Jutsu Strength passive. It also has a high chance of knockdown to trigger his Shadow Mimicry chase to immobile an oppontent.

Once you've got Hidden Jutsu +2, his standard Shadow Seam Jutsu +2, and the Shadow Mimicry chase +2, the rest are kind of up to you which order you do them in in my opinion. I haven't found the shield from his mystery to be that useful, especially since it can only be used round 2, but idk it's nice to have. The second chase that causes ignition is also nice but it just triggers twice. If your normal combo doesn't already include it, go for the mystery BT first I guess.

Good luck!

So youre basically telling me not to really use him, gotcha. Thanks for the info, it really helped :)

What? I'm not saying that? You can still use him in the way you planned, I'm just saying he's better with certain BTs. If you don't want to go to the effort of getting all those frags (admittedly it is annoying since he doesn't have skill trials right now), you can just get the Hidden Jutsu Strenght BT and be done honestly. Like I said, he is good with Kushina in the teams you mentioned so I'm not saying don't use him.