Disclaimer
This guide is written while set 10 is still on PBE. As such, a more detailed version will probably be made when it gets live, even though Mortdog said the latest patch will be the one to stay until then.
Late game
Early game
While the Superfans are usually there to strengthen your Headliner, in this comp, your Headliner is one of them ! More precisely, Gnar is meant to be the Headliner, since he's a much more agressive champion than Neeko. He also benefit from Pentakill which strengthens him further.
Superfan will give him a Sterak's Gage which will be evolved into its radiant version once you get to 5 Superfans so you only need to give him an Infinity Edge and a Bloodthirster (that you can replace with a Titan's Determination if you have augments which give your units sustain). You don't even need to put him on the frontline since, as a Headliner, he will start the round by transforming and leaping over his target, no matter how far it is.
Since you'll spend some time rolling to find Gnar 3 (make sure to check if some are already out of the pool), it's unlikely that you'll reach level 10, but if you do, Akali will be a nice addition. I recommend making her assume her K/DA form, as it'll give a little push to your team's stats (and her spell is overall more useful).
If you didn't hit Lillia 2 and Neeko 2, you can also take them out to replace them with Yorick and Viego, thus reaching 7 Pentakill, strengthening your whole team in exchange for slightly weakening Gnar.
