If you're usually running all-upgraded cards, that could be part of your problem. It consumes a lot of points. Starting at over 150 life, even better maxed out at 200, can be a huge advantage for many types of decks. Think of it as buying an extra couple of turns, which is often enough to make all the difference.
I also try to think about how the deck will play against various popular decks, and even test them out (click on the card image on the lower left hand side of the arena to play against your own deck) against the ones that I can build. Against ROL you need ways to clear out creatures (which you've got with Pandemonium). Against control decks you need to be able to wipe out permanents (you've got Butterfly). Need some defense against fast decks (you're a little light on that, just Maxwell for creature control and Discord to disrupt their game) or be super-fast yourself.
Your mono decks probably have trouble because they can't do all those things.
Best suggestion is to have a couple of popular deck builds (I use a ROL and a Dragon Rainbow) to test your arena deck against, and go from there.