I've tried decreasing my SBS. Playing naively, it was 649. When I smashed my troops getting to the top, it dropped to 625. Then I played really badly to the second gate, losing gold, sending in 0.1 dryads to fight 3 waves ... anything I could think of, and it dropped to 590. I think there may be a filter on the number, so it doesn't drop fast. But I lost a fair bit of stuff playing badly, so it makes me wonder if you are better off just aiming high and losing badly.
It's a problem with this game that it pushes back on you with every improvement, so you wonder what the point really is, outside of making your score higher.
So, now I'm fighting with SBS 590, Needles 10, Dwarven Bulwark 5, Martial 5, and I'm waltzing up to gate 1, with only a few archers lost, which I cloned again using 10 hrs of instants. Smashed gate 1 with 5 squads of light melee, with 15% boost from the Spire spirit.
I'm going to PAY for this next week, when the SBS will be 750