What mutant is used
As soon as you choose a friend as Tag Team support in your campaign (or tournament) the system picks a mutant from that friend following the rules below (from FAQ section):
- highest level among all mutants - the mutant with the highest star - if tied to several mutants the mutant with the highest XP bar - The last mutant that filled XP bar.
How to choose a Tag Team partner
If you are choosing a stage in Campaign or a fight with an opponent in Tournament you see the opponents you will face (or the graphs of the opponents gene types in Tournament). As soon as you click "Validate" within the next screen you can choose your Tag Team partner or "No Tag".
Eva and your own avatar should always be among them. You can choose Eva (or any friend) if they are marked ready.
Attention: Should you choose a friend to help you via Tag Team and you never used the Tag Team button within the fight itself, it counts nevertheless!
The system will always preselect a friend if his Tag Team option is ready. You should consider if you would like to use that attack now or save for later choose the "No Tag Team" for example.
How does Tag Team work
If you chose a friend during the section screen, you will see a aqua-colored "Tag Team" Button in the lower right side of your screen. You can use this button one time only for free (all following usages cost gold). This button is moreover only useable if one of your creatures is on its turn to attack.
Attention: Should your whole team die before your first mutant is on turn to attack, you cannot use Tag Team and this friends attack will only be available 24 hours later.
Should you choose to use Tag Team while one of your mutants is about to attack, this mutant will do its damage as usual. Afterwards before the damage to its opponent is summed up, the Tag Team attack will com in effect.
The Tag Team mutant will appear from the void, do an attack and vanish back into void. After your mutant and the Tag Team mutant did their damage, their combined damage will summed as it were a single attack. The remaining fight will continue as usual.
Attention: Should your mutant technically kill an opponent before the Tag Team mutant attacks, this opponent still gets the Tag Team mutants damage as it were still alive.
If you are using a mutant for attack which uses a multi-target attack (e.g. the Robot) you will still have to choose one opponent. If the Tag Team mutant does only have a single target attack, it will attack the opponent which you highlighted during your multi-target attack (mouse over and click).
Eva usually is more "Promo-focused" to put it that way: in most cases you only have to guess which mutant she will use (usually one which is in shop that day).
For all other friends you might know their strongest mutants and can plan accordingly. As for normal fights, attacks from a Tag Team mutant will have the same bonus and penalty rules as your own attack. Should for an example for Tag Team friend have a Leech Lord as strongest mutant, you should use him against a Galactic or Cyber attack of your own mutant, not for example against a Mythic or Saber opponent, even if your own mutant might do good damage against them.
If you do not use a tag during a match, your team is healed slightly. The health percentage a mutant regains is dependent on its level.