You know, I thought about setting low point characters in weak nodes as a general strategy because of this, but I think for most people I beat it's harder for them to have enough characters to get through all the nodes easily.
Thing is once I got a taste of the diamond tier I want it always. So far this is best defense and hard to pull off. You would need a fighter weak enough your opponent will use one of their 2nd rate carries but your defending fighter need good enough move set to trap them into either losing timer bonus or hp bonus(for trying to bb finish).
I had this happen TO me way more than FOR me. But once I was matched with someone 1 tier higher than I am with boss node 10k more. They had a balanced defense but lost twice to the single fighter node and just give up or ran out of carries before the boss fight.
From a pure numbers standpoint, the best points strategy might be going with as many Immortal Fibers as you can. They might not *win* but potentially robbing your opponent of 500 points for a no death victory, plus all the points for health seems like it might more than make up for the 100(?) points you'd lose from your diversity bonus that way.
I'm thinking evolved Grey Matters might also make a good choice on that front, since in my experience, an Early BB3 by the enemy is all the difference between success, a close call or even defeat.