While the Rock and Cena are obviously ridiculously strong, don't sleep on Seth.
AP is always building at a rate of 1 every 2 minutes. It will stop when it reaches your limit which is tied to your rank. You can over the limit with rewards and it will reset when you rank up. It is best to always be low on AP.
There used to be a system where you could watch an advertisement for 50 ap. This was limited to around 5 per day. Now if you look in the secret shop, you receive a pack containing 30ap and a skip ticket, which is very nice.
The daily quests often reward you with AP for summons or unlocking a core so be careful when collecting them from your Inbox (don’t go over your limit!)
A note about the daily quests, if you are worried about using up your AP, you can often start the quest and then just tap out and still get the reward!
Tower of Trials
Beat the levels as much as you can and you can get the same rewards every day! Note that the first reward and the repeated reward are different.
Go to Exchange > Exchange shop. Buy the Ap elixir items using Arena coins. Don't spend your League match coins, unless you don’t need that selector.
The secret shop refreshes every 4 hours and has items such as 200 AP for 10 rubies if you're desperate.
When you Rank up you will get all your AP back! However, if you have any AP under the limit, they will be lost.
Some gains to remember when ranking up:
Do not exchange Rubies unless desperate, but you can find AP in the secret shop, and of course they are part of packages in the general shop.