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Trying to do dokkan events is like jumping straight into endgame content right after booting up. You're not supposed to.
Most dokkan'd character shoot up in team cost dramatically and at your rank you won't even be able to use them effectively in team because of that.
The most important thing about building teams is the leader skill of the card you set as leader. You want to use a card that affects all cards in the team, and you'll want a leader skill that boosts both ki and stats; the best ones are ki + x and all stats + x%.
SSB vegetto is a great leader because he allows several different cards in a team as long as they are "super" types (heroes) and boosts ki + all stats.
However unlike the 120% leaders, ssb vegetto (and rosé) also boost unawakened cards in the team, so you don't have to z-awaken your entire team to get the boost.
And yes other cards will seem weak next to ssb vegetto because it's "god" tier card:
God tier
Category leads, these boost a specific category of cards, like ssb vegetto has the "realm of god" category, UI goku is the lead for this.
Neo god lead, these boost a specific type and affinity of cards for 120%; ss4 vegeta is the "Super" "AGL" lead
God leads, these boost a specific type of cards for 70%; AGL super vegetto boosts all AGL type cards.
Demi god leads, these are the same as god leads, but they have a higher boost (100%) to offset that they don't boost ki; INT ssb vegeta is a demi-god lead.
Affinity leads like ssb vegetto and rosé.
Rainbow leads, these cards boost all types regardless of affinity; masked saiyan, caulifla and arale are currently the only ones we have (that are god tier).
Old god leads, these boost for ki+3 and atk +3000 for a specific type; INT ultimate gohan is an old god lead, they aren't used as leads anymore because they were straight up replaced by the 70% leads.
All other cards are either hard hitters, support units, or trash.
You can tell by looking at their super and passives, the hard hitters will generally do supreme damage and have a passive that raises their attack by x%.
The support units will have a passive that affects the battlefield/allies somehow.
"Best" cards is also kinda not always the best option for the team, you want teams that synergise well, or in other words, you want cards on the team that have a lot of link skills in common.
For example, SSB vegetto has super saiyan, prepared for battle and over in a flash. Putting him next to a card that has those same links will suddenly boost him with +5 ki and +10% attack.
And with links come rotations.
The two characters you want to keep together are the ones you keep in the first two slots in battle, the way you put them together in the team building menu has no effect on rotations.
This way those characters will always show up together every other turn, the third character is called a floater and is generally just there for support, like change orbs, give ki or boost stats; they only return every 4th turn.
After building your team, always pick a friend card that also has a similar boost; it's pointless for instance to build a team of AGL cards and then pick a friend that boosts STR cards.
Just go through the story mode as fast as you can, this will rank you up fast and you'll get hundreds of dragon stones.
Use these stones on multisummons on the UI goku banner, because it's the best banner we've had since ever basically; this banner is live till 6 jan.
Ignore the other banners.
Also, never ever ever ever use stones for continues holy shit, you'll get much more out of them by using them for refilling stamina instead; they are also not farmable, only the story, events and login bonuses give them out since they are in fact premium currency.
Do not spend them on continues.
Also you don't have to z-awaken every single cards, that's just a dumb drain on resources.
