If Angel’s Tomb is already a creature when a creature enters the battlefield under your control, its ability will override any effects that set its base power and toughness to specific values, but other changes to its power and toughness (such as the one created by Titanic Growth) will still apply.

Effects setting Angel’s Tomb’s base power and toughness to specific values that begin to apply after Angel’s Tomb has become a creature will override the effect of Angel’s Tomb. For example, if an effect causes a 3/3 Angel’s Tomb to become 0/1, it will remain 0/1 until another effect (such as triggering the ability of Angel’s Tomb a second time or targeting it with Titanic Growth) causes those values to change.