Get a *6/60 Campaign farmer. Kael or Athel will work nicely. I prefer Athel, but that is just me. Advance to Brutal 12/3 (slightly more silver) or 12/6 (slightly more XP - 8800 per champion with a farmer + 3x food).
Max out your Gem mine. It will pay for itself in 100 days. It means free 15 Gems per day since then, in addition to those you got back during the 100 days.
Be sure to unlock as many Market slots as you can. You may find a lot of useful things there, even Ancient shards from time to time. Buy as many Mystery shards as you can.
Be sure to unlock as many Sparring Pit slots as you can. It is free XP.
If you can, use double XP and farm, farm, farm. It takes 10 runs in 12/6 to get three Uncommon champions to level 20. It takes 23 runs in 12/6 to get them to level 30. Total energy cost is 264. As soon as you level up your account some, you will be able to get three level 30 food champions every day with just your full energy and the two free daily energy refills (60 minutes play time and 5 Classic Arena battles).
Plan ahead. You will get some free energy refills in weekly and monthly quests too. Save silver for Artifact events/tournamets and lucky Market offers (like Speed boots). If you can save some energy refills or energy in your Inbox, do so. Use those during events/tournamets.
Do the quests, missions, and challenges. Advance the Campaign and Faction Wars, you will get nice rewards. Try to put up at least somewhat decent Classic Arena team, and 3 teams for 3x3 Arena. Any rewards count and participation is free.
Join a clan, attack the Clan Boss as soon as you can. You will get some nice rewards every day.
Do not waste your Void or Sacred shards by openning them outside the quite frequent double pull chance events. Screw the 10x chance events, those do not help with champion rarity at all. Maybe, just maybe, open your Ancient shards during the 10x events, you may get some nice champion, but do not expect much more than a rare due to the rarity pull chances.