Missions you have already completed requirements for will be automatically completed once you finish the one you're stuck on. There a few exceptions though ( potion mixing and ascention missions come to mind ) 

Ya, Excepy anything to do with Arena is Extremely (( Inputs Deleted Explenative here ))

I'm confused as to the complaints about arena, the bots made it farmable at all levels correct?

Yeah I was stuck for a month trying to get enough medals to get a mastery to 10 and this was before bots.  I don't see where the complaints are coming from now. 

The complaint is bronze arena broken teams at 100k plus teams using hacks just got raped by 37k team doing 175k damage single shot against my team of 6* I don't use bots because I don't cheat or risk banning I get to bronze 4 and that's it. I 3 stared brutal auto fight dungeons to stage 12 and I can't get silver 1 and the rewards are good after a month of this crap something needs to be fixed.

Proof 

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When the other guy said bots, he wasnt suggesting that YOU use some sort og bot, or cheat or hack.

You will notice sometimes when you refresh enemies in the arena that you will get a whole page of easy teams. Sometimes they even look difficult like 4 lvl60 legendaries, but may only have like 30k power or something, so even those can be beat.

The other thing to maybe look at is - who is on your arena team? What is your team style/strategy?

As long as your team is reasonably sensible, you can probably get to gold 4, just by focussing on the easy enemy teams. There's really no reason for you to be stuck at bronze 4 at all.

I'm not sure there are bots in the lower levels anymore... hearing more and more complaints about bronze arena again. And even up in gold IV, it has become rarer to have a full bot screen - more often I get screens that are mostly easy but with some tougher fights, so it's less clear which are real opponents and which bots. 

I have no clue about the lower levels, I think the bots are tougher especially as you get closer to platinum. I am using a weak defense to stay in lower gold 4 now hoping to get easier fights especially with the mono-faction tourney.  I have no good team for this, lol