Nah, the worst thing is doing the mission for winning a DT in Free Fight, which we of course can't do in tourney mode because people who are trying to win the DT would hate us for trying to win a DT. Nothing makes them madder than people trying to win a DT.
No. The missions that are nigh impossible are 7 rounds with Diego Cr in Survivor Mode, 2 KOs with Mac Hen in ELO mode, etc. I've been playing Free Fight for quite some time to grind out some idiotic KO missions. You'll get people who have stupid hands in one way (my personal favorite was Aamir, Lamar Cr, Lamar Cr, Terry Cr...) or another (Frank, Gran Vista, Tafa, Mickey T). Some missions will breeze by. Others you'll have to work your ass off. No mission is impossible, but a lot are improbable. Some will be easy (50 rounds with Wendel), some will be hard (40 KOs with Rebecca). Some will be quick and smooth (7 KOs with C Porkks), some will be very long and tedious (250 rounds with Cassandra). Some are well worth it (35 KOs with Dudley Ld gets you 5 credits), some are not-so-worth-it (250 damage with Tanaereva Cr will get you 10 BP), some are semi-worth-it (250 damages with Gork will get you 2 credits), some are overly-worth-it (30 damages with Naja Ld will get you 5 credits). No mission is impossible. The ones you can complete in Free Fight are the easy ones. The ones you cannot are the ones you have to worry about
"For example, wining rounds... Am I supposed to play full Sakhrom to win 200 Rounds With Stella, and after that, play full Vortex to win 150 Rounds With Dregn, and after that something... That is just too much..."
Such misisons just take a while, just try completing those missions because you have to use stella in a deck, plus those cards can be easily be the 5* in your decks and you can add more 'decent' mission cards next to it
I just try and make decks for Free Fight with characters of the same clan that have their own missions or help out with clan missions (like gain life or block damage) and I change decks regulary so I focus a few days on missions for one clan and then go on to the next clan... Some missions really take a long time and (like winning xxx rounds with the same character since it depends very much if you have it in your hand or not), but that's just what mission grinding is all about.