Why do Gollums always lose at Fortnite Endgame?

There's a mission where you have to win 3 times with the gollums, but the gollums always lose

I played 10 rounds with the gollum and always lost

but every time the avengers win how to do that?

Because the Avengers are simply OP (mMn) …
You definitely need a full squad to have a slightly higher chance, since all the random ones are mostly Dullis, NoSkins or similar…
Most "pros" / football skins are more likely to be found in the Avengers.
For these reasons, I no longer play the mode…