Try with investment.Amarusu needs about 200 defense if it wants to survive a bullet punch. Has anyone said anything about the ability that halves damage from contact moves? I can see that making some really good physical walls. The Normal-type that is confirmed to get it essentially loses its Fighting weakness.
Edit: Nope, with uninvested base 100 HP and base 200 def, Bullet Punch is still a guaranteed OHKO.