Personally, I think Requiem of the Gods is a pretty good arena - there's no defending involved (from what I recall), some great heroes and spells, and some cool boss monsters to fight. Only significant problem is the huge map size (it's about 3.8MB I think) but there is a Lite version available (not sure of the filesize).
There's also 1v1 and team duels available (you are closed off from all other players during 1v1 duels so you can be interrupted) and there's the option to only do duels (absolutely no PvE available if you choose this mode) which is pretty cool since you get to play as most of the heroes in the game, and both players start at a specific level designated by the host, but it's been some time since I've last played, and a number of these features may have been removed/changed from what I remember
Hulk's Heroes is pretty fun as well (it's made by emjlr3, and has some cool heroes and spells, no defending unless being nice and helping allies counts as defending), but I can't remember where to download it
I just got to watch the video because even though I was there I was trying to work out tech problems with the chat since I was the social media guy so I was distracted but check it out guys the video is super fucking great!
Looks like Google fixed whatever mistake that made the recipes on the site go crazy and we are no longer trending towards a recipe site lol - I don't care though because it motivated me to spend alot of time on the site improving it and at least now the content people are looking at is not stupid and embarrassing like it was when I first got back into this like 5 years ago.
They also were digging threw old shit at the sheriff's office and I tried to get them to give me the old electronic stuff, but they said no. They can't give it to people because they might use it to impersonate a cop or break into their network or some shit? idk but it was a shame to see them take a whole bunch of radios and shit to get shredded and landfilled