So, if you're here, you're probably interested in how to get early kills and snowball to victory.
This is for Fire mains, but others c*so benefit significantly from it.

Very Early Game:
Basically right after the tutorial and elite instances open up.

Do not waste time doing anything else. Start up the elite instances and get through all of them ASAP. First time clearing everything will net you enough fragments to get a 2* Team 7. They work well together and come made to combo into each other.

How to magatama them:
If you do not have enough magatama to boost everyone completely, and you probably won't, here is how to allocate them.

Main: Anything you can spare - Usually gonna be your off tank
Naruto: Life, Defense, Resistance - He is going to be your main tank
Sasuke: Ninjutsu - He is going to be your main damage
Sakura: Attack, Ninjutsu - She is going to be your only heals for a while

After this, make sure to download the miniclient for some rewards. Included in those rewards are 2 seals for the seal lottery. Incase you got some scrub in the free seal draw, or even if you were lucky and pulled a 3*, use ONE of those seals for a 3* confirmed.

I have seen the most common various draws being: Karin, Zabuza, Shizune, Kakashi.

It doesn't really matter, but it should be obvious which role your 3* plays. If it isn't ask around, the community is fairly helpful.

Feel free to replace any of 2* Team 7 with a 3* counterpart, after checking how well the 3* comboes with the rest of your team. You can change around your main's talents to better chain together comboes.

Mid Game:

Here you will start to see people using damage carries, aka glass cannons. In battles against them, they are who you want to click on and get comboes on. Killing them will make the rest of the battle considerably easier.

This is also where you can start buying fragments from the shops. The 2 to focus on are Kurenai(Arena) and Kabuto(Survivor).

Here is where you can do another comp switch. Once you pass all the level 30 elite instances, you can get Hinata to 2* and after a week of being on, you can get Kakashi. They will probably be your first comp that can get comboes exceeding 5.

Kabuto is a good replacement for Sakura, Hinata for Naruto and Kakashi for Sasuke. Note: This is for high single target burst. There are different optimal comps depending you the strategy you choose to take.

The key is to wipe your enemies damage before they wipe yours. Make sure to stay off auto and target dps in Arena. Ai is bad there.

Later:

Basically play how you want to play. You will need some form of damage mitigation, but beyond that, you are free to do basically whatever you want.