Rustrazor Butcher

First strike is a static ability that modifies the rules for the combat damage step. (See rule 510, “Combat Damage Step.”)

If at least one attacking or blocking creature has first strike or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.

Giving first strike to a creature without it after combat damage has already been dealt in the first combat damage step won’t preclude that creature from assigning combat damage in the second combat damage step. Removing first strike from a creature after it has already dealt combat damage in the first combat damage step won’t allow it to also assign combat damage in the second combat damage step (unless the creature has double strike).

Multiple instances of first strike on the same creature are redundant.

Wither is a static ability. Damage dealt to a creature by a source with wither isn’t marked on that creature. Rather, it causes that source’s controller to put that many -1/-1 counters on that creature. See rule 120.3.

If an object changes zones before an effect causes it to deal damage, its last known information is used to determine whether it had wither.

The wither rules function no matter what zone an object with wither deals damage from.

Multiple instances of wither on the same object are redundant.