I must agree, I’m in D league and I can not win any battle. I’ve been stuck on D league for a while 3 weeks. I’d love to have the same type of players or stats as me but these are killing me and they’re points are much much higher than mine. I’m trying to level up to see if I have a shot. See what happens!!
No, you cannot go back to lower arena bracket. But you can remain on purpose in your current league for a long time, if you want, so you don’t find yourself overwhelmed again in the next one. Just keep your team power level lower than the point in which you advance. I linked the post showing the power level for each arena league.
Keep in mind that every buff to your heroes increases their power: leveling up, upgrading gear, upgrading active/passive skills.
You can level up your team to just before you get sent to new league and then stop improving that specific team for a while. Meanwhile you can work on your pets (they do not contribute to your team power), work on additional heroes you can use in dark rush, or just save a bunch of resources so you can quickly upgrade a lot once you do decide to progress to next league.