I'm still short on upped cards and thus have to stick to mostly unupped decks and I keep returning to Grabbix. It's got so much going for it:
1. It's extremely fast. I enjoy epic matches, but since Elements involves so much grinding to make money and gain cards, speed is (in)valuable.
2. Since I only have two (quantum) towers, a deck like this is the only one that allows me to play creatures and deal damage on the first turn.
3. With good draws it's possible to get out 20+ damage by the second turn (and kept unchecked a ton of damage soon after).
4. Relying only on the mark, novas and immolations for quanta, it doesn't care about earthquakes, devourers and discords aren't much trouble and even black holes aren't game stoppers.
5. I usually play a 32 card version with a fog shield (and deflag, see 8.) but if I expect steals I don't bother playing my weapon and shield, neither are necessary to win.
6. Phoenixes are tough to kill.
7. In general, having a lot of fairly powerful creatures out (Phoenixes and Shriekers) means even Maxwell's demons or otys aren't much of a threat... burrow shriekers if you must, otherwise just keep them out.
8. Upgrading: Obviously, cremations are really helpful, I now have 5 of them. Which allows me to add at least one deflagration which is nice to have for an annoying shield (fire or dim, theoretically thorn carapace). Elite graboids are better, too, obviously.
9. Weaknesses: Sure, plenty of them - depending on the draws. The good news is, especially with the upgrades, it's not bad for farming silver and even gold league because there's always the chance the draw goes well for you and badly for the opponent and it's all over in 6 or 7 turns. The bad news - it's not consistent enough to win six in a row in bronze for a special spin and there are "safer" options for AI3. Grabbix's nightmare is obviously... nightmare.
But who doesn't hate nightmare? Oh, and the unholy shield, which fortunately not many people use. The fewer HP a creature has, the bigger the chances of being turned into a skeleton, so it's the perfect counter for phoenixes.
My humble summary: Speed is probably your best bet in the arena, where opponents have more HP, upgraded cards and so on. And speed is good anyway unless you want to waste the whole day playing Elements. And considering the setup it has surprisingly few weaknesses. And it's flexible, too. I've played with a vampire dagger, a nightmare, a purify... take out one or two cards and add others instead.