A little bummed to see season 3 start immediately. No time to try and fix my MMR somewhat with lightning rounds. It will be a bear to make the 3000 point Falcon, let alone the heroic 10 pack.
My new method of dropping MMR is working to some extent.
Whatever PvP event you are going to play, add a three hour buffer to the beginning - either start three hours earlier or plan to play three hours extra.
Start the event and roster wipe with the free character and everyone but your primary characters (this includes your version of the free character), starting with your 1 and 2 stars. Don't wipe anyone that's featured in something else (like Mohawk Storm in PvE), either. By the time you get to your tougher characters (85s you don't use anymore, 3*s you're cultivating, etc.), some of your first fighters will already be into healing mode instead of downed mode. Send them back in to get smacked.
Don't retreat. I don't think that does anything anymore, or at least, not significantly. Also, use this as an opportunity at 'free' skips. If a node gives you a fight you know you can win with your main team, especially if you get one of the 50 point matches, leave it alone to come back to. Nobody will be attacking you to ruin your board, as you don't have a defensive team yet.
You'll know when it's working, because you will start to see teams that are in your same situation (not quite 141 3*s, less than max featured character, etc.). Go a little further by a couple of matches to offset your forthcoming wins. Your MMR should now be manageable for the initial climb in this tournament.
The only variation I put is if I can get the seed teams. At that point, I earn my way up to about 100 points or so, then beat a seed team with a tank team. I know tanking doesn't do much anymore, but even a little bit helps.
SHIELD Training and Lightning Rounds give a free chance at lowering, too. In SHIELD, I don't win any at first, just feed some roster wipes into the woodchipper for about a week. In a Lightning round, I get some ISO and a token, then roster wipe at least more times than I won to seed my overall MMR.