to advance, do nothing except for grind campaign to build food. the fact that you've made it so far in dungeons without any 6 star champs is impressive, but also a terrible approach to the game. don't hit any dungeons until you have at least 3 or 4 champs up at rank 6.
in regard to your team they are, shall we say, not good. the low rank is a big part of that, but there's also no cohesion and all of your champions are badly built. i'm assuming that by team you meant for arena, so we'll analyze it from that perspective. you have two champions whose job is to deal damage, but nothing to support them in doing that. no tm control, no AoE decrease defense, no atk up; you're just relying on the raw damage of your champions. rector drath and sepulcher on the other hand bring absolutely nothing to help offensively and aren't effective enough on their own to keep your team alive for an extended period. you really want a team where all the champions work together and build on each other's abilities and this composition just doesn't do that.
now to their builds. the biggest issue here is that you're prioritizing sets when you should be prioritizing stats. getting a set bonus is nice, but it's far more important to ensure that you're using gear with the right main and substats. for a dps champ like ronda you'll want crit rate or crit damage gloves (at your stage in the game, you'll likely need crit rate to ensure she reaches 100%), atk% on her chestplate, and speed boots. for someone like rector drath, you'll probably want hp% or def% gloves, hp% or def% chestplate, and speed boots (barring a few extremely specific builds, every single champion will want speed as their boots' main stat). substats will be similar. ronda will want a lot of atk and crit substats, drath will want more hp, def, and probably some accuracy for her a1. now, if you can get gear with good stats and achieve a set bonus as well that's great, but for the moment you should just use the best gear you have and if it's from half a dozen sets, so be it.
(in addition, the sets you have chosen are awful matchups. sepulcher and drath are both support champions and both are in sets that rely on high damage output and are meant for dps champions. tallia is in a set that requires high hp, one of her lowest priority stats. ronda is at least getting two different bonuses rather than having 6 pieces from one set, but one of those primarily boosts accuracy, a completely useless stat for her.)
oh, and you can show your whole roster by hitting the button in the upper left that looks like 6 dots next to a line.
