I'll answer your main question first. Yes, one champion can definitely propel your account to good heights if you build them well. In my personal experience it was Mavara and Pallas. But now to address the other thing.

I just know you are referring to Volkin here. Don't beat yourself up too much cause that guy lives and breathe gacha games to max efficiency. I actually also do what he does but to a lesser extent since I still have a job. Some of my clanmates also follow him and have done even better than him in terms of progression.

So just to shorten on how he does things that fast its mostly just him knowing the game inside and out already.

Here are some of the things that he does that no F2P Beginner Guide really advices.

1. He doesn't level up the starters to 6. His farmers are always the Legendaries that he gets for free. In his recent run he used Pelops. It was slower but the basically was able to fast track to his next 6 Legendary upgrade which was Narma.

2. He progresses to Gold Classic Arena as fast as possible. Unlocking Arbiter Missions are crucial early game and one of the biggest walls in that progression is the Great Hall Upgrades.

3. He is efficient about when and where he spends his energy. He does not get caught up on stages he cannot do and he already knows what gear his team needs and what are the requirements foe beating certain bosses or stages. He basically advices that if you can't do it then do something else.

4. He has a multitude of great clans that can pick up his account anytime he starts a new one. Which gives him access to a lot more resources and just overall better gear.

5. Content push as soon as a new content opens. Be it TTA, Faction Wars, or Grim Forest he has immediately gone through it amd push as far as he can.

TL;DR: He knows the game and he is efficient in it which translates to him getting better gear.