Arsenal Thresher does not have a -1/-1 counter on it.
Steps
Activate
Cauldron of Souls by tapping it, giving
Arsenal Thresher persist until end of turn.
Activate
Krark-Clan Ironworks by sacrificing
Arsenal Thresher, adding ({C} magic symbol) ({C} magic symbol) .
When
Arsenal Thresher dies, its persist ability and
Dross Scorpion trigger.
Resolve the
Dross Scorpion trigger, untapping
Cauldron of Souls.
Resolve the
Arsenal Thresher trigger, returning it from your graveyard to the battlefield with a -1/-1 counter on it, and then choosing to reveal at least one artifact card from your hand to remove the -1/-1 counter.