So in game lore can we assume frontier brains are at least on a level equivalent to their region's Elite 4? Maybe stronger?
It's obvious why they use legendaries while other trainers don't, as it would be unsportsmanlike for gym leaders, the E4 and the champion to confront challengers with such extremely rare pokemon. That doesn't mean leaders/E4/champs can't catch them.
On the other hand frontier brains don't have to give a fuck since their whole thing is just to be as strong as possible.