[quote=@RokkuHoshi]
<Snipped quote by Lugubrious>

Well, I would guess the ring is 15 meters. That's 25 blocks including the center. That takes me all of 15-30 seconds to fill in. Considering Steve is a master builder, 10-15 seconds.
[/quote]

There's not really been any indication of ring size, though since you specifically stated that Steve's blocks were each three meters by three meters, it would only take five to span a 15-meter arena.  Still, even if being a 'master builder' -which isn't stated on his sheet beyond a strength being 'building'- allows him to place blocks faster than usual, it takes less than three seconds for Slayer to strike twice.

All that aside, since it takes an ordinary minecraft player ten seconds to break a cobblestone block by hand, it doesn't seem to me that someone like Slayer with superhuman strength would have much of a problem breaking any blocks that fall toward him, if he even deems that necessary.