in 2 weeks u can do 2 Colosseum, then u have all u need, and of course, i'll save epic/legendary/mythic chests in the tower for the event.... u can believe me or not, but that's it. ah, u know in the dungeon EVERY monster count not only specific... yes? then, if u must do quest 18 Defeat epic monsters in the Dungeon rooms 20 times - CD 1h30m = 3 passes (1free)=4h30m or 15 epic dolls u need not 4:30 hours but 5 mins..... and NO ONE epic doll....
obviously I do the same, and yes WITH help of Colosseum event its passable (still not in 1st day), but what can do new player with Basic sets, and maybe few Event set, up to Legendary cards? He can't pass Colosseum, just cuz he will is run out of cards somewhere in the middle (if lucky enough).
And no, if you need to beat Epic Dungeon monster, that means you need to beat Epic ones (5 of them), others not counts. You didn't know that?
Only very 1st quest of that type allows to beat ANY monster, others is rarity specific (Rare, Epic, Leg and finally Mythic (quest 43, with CD of 8 hours))