Ah yes, the classic slow rush. A slow rush loses to a real rush and to the good stall decks because it's not fast enough and gets whittled down and outhealed. Luckily AI3/AI4 don't have good stall decks, but they can rush from time to time. I imagine this is why you're considering more survivability. You're aiming for AI3/AI4 with a fully upped deck that isn't optimized for pure speed, which as others have mentioned isn't optimal, but let's take the challenge for the sake of fun.
Between Gravity and Death, you have various ways to trigger death effects for Bone Wall, or you just go with random Shards of Sacrifice. Sadly the latter tends to work, and would be the better choice against AI4 because you invest less cards. AI3 on the other hand does piddly damage that BW can stop without even adding any CC (creature control).
If you only want a bit of defence against AI4 too, Bone Wall+Elite Otyugh is a very classic combo. The other option is Bone Wall+Gravity Force, which naturally comes out earlier and has less weaknesses, but is not repeatable. Since your deck isn't terribly slow, you will get by with BW+GF since you can close up a game before you die. If you only pack 2 copies of BW and GF, you can get by on 3 Guards, but the more you invest in the combo, the more Guards you'll need to make it consistent.
Naked BW's are an option against AI4 too, but I would rather use a few SoSa's.
