Rush-down was the safest move (and how a driller fixed it)
When the best strategy is to skip the game, change the rules, not the numbers.
Sapper is a cave defense six floors deep. Before build 12, you could drop to the next floor any time. Kraig's playtest showed what that meant: rushing down was always the safest move. The less of a floor you played, the less of its swarm you had to face.
That's a design bug, and tuning won't fix it. Make the swarm slower and rushing is still best, just less so.
The fix: make going down the fight
Build 12 added the Driller. It's now the only way down. You build it from what you've mined, and while it bores the swarm hears it and comes in bigger, faster waves until it breaks through. Your squad guards it with you, and drops with you. Anyone who dies stays dead.
- Going down now costs something: ore, which means mining, which means staying.
- The hardest fight of every floor is the one you choose to start.
- Deeper rock needs a better drill (Mk I, then Mk II with copper, then Mk III with carbide), so every floor asks for something from the one before.
The general rule
When players find a way around your game, ask what they're avoiding and why it's worth avoiding. Then put the thing they want behind the thing they're skipping. One rule change did more than any number of tweaks to wave sizes.