Fully maxing one's Gem mine should ne a priority for anyone who plans to stay for a while. It will pay for itself in 100 days, and sicne then, it is free Gems every day. Speaking of Gems, the daily Gems packs that lasts a month is one of the best packs you can buy.
Unlocking all Market slots will allow you to get some nice gear, Mystery shards, and rarely, an Ancient shard too for 200k silver.
The offers do differ for each player (also, the more you buy, the more expensive packs you will be offered), but in addition to the daily "Rookie" pack with a single Ancient shard, I sometimes buy the "Mini Max" pack too, containing a an Ancient shard, an energy refill, a one day XP boost, a Classic Arena token refill, a Clan Boss key, and 30k silver.
One important thing concerning shards is to save your Ancient or better shards for 2x rarity chance events. It is tempting to open them immediately, but the double chance for an Epic or a Legendary is not to be underestimated. Mind, that Void shards are essentially the same as Ancient (blue) shards, but only producing Void affinity champions. They are not some kind of "Epic" shards. The chances are the same as for Ancients.
I am now almost happy with non-Void Rare duplicates, which I can immediately feed to my food champions (or use them as such), saving me a lot of energy.
A Christmas fusion is on the horizon, and that means a HUGE LOT of energy if you want to participate. Newer players may not be able to, due to the lack of the champions needed for the fusion, though. Fusion is not only about levelling the champions up, but also about a wagonload of potions. That means even more energy, or a lot of real money.
As for Athel, she is still my Campaign farmer even at account level 70. She will help you a lot in early Arena and in dungeons too, especially in Fire Knight (3-hit A1) and in Spider (her AoE hits like a truck). I find her awesome.
If you are not in a Clan, join one. The daily rewards for Clan Boss are not negligible. Really nice sets drop there. Sometimes books and shards too.
Put together three Tag Arena teams, no matter how strong or weak. Try to have at least one team a bit stronger, so that you can start collecting gold bars. You can get a very good Legendary there, Drexthar Bloodtwin. It will take you a long time though. Stay low for starters, leave only some solo level 1 champions in your defensive teams to prevent getting promoted, becasue the lowest tier teams will often offers some easy wins, preventing a waste ot the precious tokens. I do that too. It also helps new players to score some guaranteed wins and to get some gold bars.