Well, I think the active player pool in the game is quite low. Maybe around 500 people? And as far as I saw there are currently just about 4-5 whales (players who make heavy use of real money transaction) so there should be a fair chance for top 50.
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So, the hard arena bosses are single element teams of one UR and 4 SR, max evo/level and sporting 4 copies of their awake skill. Since the boss you fight is random it's impossible to work on element advantage or an individual strategy, unlike Hug in last event.
So in general what you can do is make the strongest team you can and try. There are a few points to keep in mind:
1) It's kind of obvious, but don't expect to win consistently without UR cards of your own.
2) In 5 vs 5 format battle, speed is the most important statistic. If you hit fast you can get a numerical advantage early and face less retaliation. So try to get the fastest cards and speed buffs in your deck.
3) Also obvious, you want 4 skills on every card to have 100% skill rate because basic attack sucks. Since awake skills are useful, before fusing a copy of a SR/UR you want to level it to the max so you can transfer the awake skill (Except maybe buff skills). You want to hit fast and hard so the best skills are the 4 - 5 targets ones like those of Fenrir/Singha/Pramzius etc.
4) Cards of the element of the day get x1.2 statistics, so favour those when making the deck. Full single element teams on the other hand are dangerous because you may met the boss with the advantage on you. Unless you're confident in your single element (ex you have various UR) you'd want to include at least a counter card of the other element.
5) Even with everything else prepared, you might lose because of sheer bad luck (especially against Vouivre team sice she and Pramzius have dodge passive). If you can't secure 5 wins out of 6 battles consistently (the break even point) you should just play medium arena bosses.