Fwiw, full contact foamie combat, even with planes besides zagi-types, can be, and has been done safely, I don't think that's the issue. In fact, I think there was a write-up in the AMA magazine about a "last man standing" type of bout during one of the big indoor events. The winner was one of those flat-foamie bipes. So, done right, no one thinks it's not fine.

But the origional post didn't mention the size of the planes. It didn't mention electric vs fuel powered. It didn't mention much at all besides intentionally crashing planes in to each other. And some of the follow up posts from the origional poster almost implied that doing things like buzzing pilots or other people was somehow ok or part of the game.

The key thing, if you want to do "last man standing"-type stuff is to make sure of a few basic safety rules/guidlines/whatever-you-want-to-call-it

Create "setback" or pilot protection such that a disabled and crashing plane is highly unlikly to hit anyone. All you really need is two lines on the ground. One that is "no person beyond this point while combat is in progress" and the other is "no plane closer than this point while combat is in progress". Place the lines a good distance apart, and you're good there. ("good distance" is going to be a judgement call. For small foamies, ~50feet might be enough, might even be a bit more than enough).

Agree on a weight / power / airspeed limit of some sort. You don't want someone flying an 6lb (roughly .40 size) electric fun fly against small foamies, etc. Since you're just messing around with friends, this could be as simple as a verbal agreement about what planes you're flying.

Make sure spectators / buildings / cars / anything-you-might-damage isn't any closer than the pilots to the planes. (see the setback above). It's bad if you hit each other, it's a lot worse if you hit someone else or someone else's stuff.

You get the idea. Just remember that full contact combat does bring with it some extra risk, and that screwing up can cost you and others.

For example, a site that is safe to fly in might not be safe to combat in. And if you fly combat there anyway, and something goes wrong, you might find yourself not allowed to fly there anymore. Certainly, a land owner or local government isn't going to listen to "we promise to not fly combat here anymore, but can we still fly around?"

(I saw local government because in some areas, some towns/cities have already passed laws the prohibit flying RC planes in their parks, and I've heard of at least one town outlawing RC boats as well. It might not happen to you. But it might).