Considering that champions will have different areas of the game where they shine most, anyway... I don't see how you could have a single method of testing out any new champion. You'd have to test them in the area where you think they'd work, in a team where they seem to make sense, which can be different for each champion. Of course, you could also look them up on specialized websites or content creator's youtube videos, to see what the results of other people's testing was. 

And sometimes you'll get new champions who are very good and might be a gamechanger for other players, but just don't fit into your plans or only cover ground that you already have other alternatives for, so you end up not using them much. 

Most champs have a specific role according to their skills.

If you look at Youtube videos, many CCs test champs for Damage and have their own standard setup for it. The other abilities can be quantified rather well by looking at the skills and their cooldown times.

Peydma for example can hit pretty hard. I have her as a main champ on a secondary account. But aside from hardhitting, I find her ability to steal buffs much more useful.

If you want to have an impression about the abilities and strong sindes of a champ, I would recommend looking at various websites (not only one) describing the champs and their role. 

Naturally you can set up your own testfield...but you should than regard the different roles. Arbiter most likely will not be able to ountnuke a Foli or Trunda...her role is different and at least equally important. There are many nukers....but only one Arbiter....

If Peydma isn't usefull for you at other stages than Magic Keep, your account is either really, really strong and you wipe out dungeon waves with your Royal Guards or Elenaril/Taurus/Dracomorph on first sight, or your account is rather weak and you can't fight high dungeon stages, where the waves hit back, at all. Peydma gives a constant atk down debuff for every enemy during the whole fight. That's usfeull everywhere in the game.

Like L9753 allready said, sometimes champs cover ground you already have other alternatives for. Maybe you know that atk debuff is helpfull at higher dungeon stages and especially at the Clanboss and you allready have a Stag Knight, or a Dhukk, or somebody else for that job.

Two great options for you:

1. Doom Tower gives you 100+ floors of varying difficulty to test your champs for free in different comps as many times as you want

2. Tag Arena a great testing ground as well, while not free I imagine most are not using all the tokens.

Hey, this can also end the complaining that I can't play Raid because I need energy!!!  

Would be good if players could run their own roster against each other at a cost of no energy and no reward in order to tune their teams

Would be good if players could run their own roster against each other at a cost of no energy and no reward in order to tune their teams

Thank you, creating such a game area might be an interesting idea. It is not possible to promise anything certain now, but we could ask game-designers' team about adding some feature for testing champions in your roster.

Guys, if you envision some more details about implementation of such a feature, please, feel free to share them here

It would also be great to use an idea such as this for testing new gear. In addition to the fitting room (which is awesome, thanks) it'd be nice to be able to outfit champs and test them out.