Scorched Beast Queen and mutated events with reflective are your best options for events but most likely someone here will help out. Don't need to be in a group, any poor soul can be revived.

Originally posted by Arnold Ziffle:

Scorched Beast Queen and mutated events with reflective are your best options for events but most likely someone here will help out. Don't need to be in a group, any poor soul can be revived.

Thanks.

Originally posted by Alejanthrox:

I need to revive an ally but I don't have the opportunity. The people in events are very high levels and do not die.

I suggest to try the "Encryptid" public event.
That one is very popular, as it can potentially reward the plans for the popular gun "The Fixer", so there are always a dozen or more players.

In addition, the boss enemy is very powerful and even invulnerable for most of the time, so it's almost guaranteed that there will be someone getting downed.
(By my judgement, players mostly stay downed and don't immediately respawn, as there's the risk of the event finishing before they are back and within the event zone, missing out on the rewards.)

Furthermore, there are some sizable water puddles which count towards the similar challenge "revive a player downed in water".

Alternatively, you can always ask someone through voice chat

Originally posted by Alejanthrox:

Also, I don't know if it is necessary to be members of the same group. Someone help me? thx

During events, all players within the event zone are considered allies.
Outside of events in the open world you need to be in the same team to count as ally.
Same goes for expeditions and daily ops.