While "what kills you" varies depending on place and time there is one thing that kills you more than anything else. Artillery. It's crude, simple and effective. Throw something that blows up at the enemy from far away. Throw lots of it. And we got really good at it. Depending on theater and terrain. In the european theater artillery (and mortars) was responsible for 2/3 combat deaths in ww2, in the mediterranean it was even more. In the pacific which is a little less well suited for deployment of massed artillery the figure was still close to 50%.

In very general terms, the more heavily wooded a battlefield is, the more impact small arms will have but even in the most dense jungle you're still more likely to die from an artillery shell than a bullet. CAS works too but building a plane is just so much more costly than building a cannon so they were never numerous enough to seriously compete in terms of numbers. Strategic bombers aren't really a battlefield weapon (although they deliver quite the punch) and tend to kill civilians rather than military personel.