There's been a lot of concern about the lockbox task requiring a score of 70,000 on the Thanos boss in mission 3. I'm here to tell you that you don't need to panic. You need to look at this with math and relax. So, let's just get straight into it.
Fight Points Explained
If you take a look at the fight score system, you'll see that we can work the requirement backwards. By doing so, we should really take your stress level about this task from about a 9 way way way down to a 2 (on a scale of 10 being can't breathe, need intravenous Xanax and 1 being parked on a beach somewhere, under an umbrella with a fruity drink and a good book).
So, you need 70,000 points. You're going to do this as a three of a kind fight, Thanos with Proxima and Corvus. That will give us a x15 multiplier. 70,000 / 15 = 4667 points. Much more manageable, right.
It gets better. You're going to have all three heroes survive the fight. That produces a x3 multiplier. 4667 / 3 = 1556.
It gets even better. You're going to win the fight. That's worth 500 points. 1556 - 500 = 1056.
Yep, it's better than even that. You're going to fight three bodies. That's 450 points. 1056 - 450 = 606
You are not going to rest, you will use an item if you need stamina recharge. That's worth another 150 points. 606 - 150 = 455.
Now, to get 455 points, you need to survive the fight with something like half your life, and do so in 3 rounds or less. Any combination of remaining life (1-500 points) and time (0-500 points) can take you over 455. But, in case you miss, you can get the remaining points through Team-Up Bonuses, through overkills on Thanos (the other two cannot be one shotted nor overkilled since they flee) or through the Quartermaster bonus (5 consumables used in the fight = 125 points, every additional consumable produces an additional 25 points, to a maximum of 250 at 10 items).
There are lots of ways to do this, but if you want to be extra sure, you could run Black Panther (150 points), leaving you 305 points to get. If you use some items that get you extra turns (i.e. Chrono Accelerator, Chrono Boost, Holographic Hand-Link), you can cut down on the number of rounds you use, maybe to 2 total rounds, netting you 300 points. As long as you don't have anyone die in the fight, you're fine for the score.
Relax. This is easier than it looks. Winning this fight in two rounds will generally get you the score you need.