Flayer of the Hatebound does not have a +1/+1 counter on it.
Steps
Activate Sensei's Divining Top's second ability by tapping it, causing you to draw a card and put Sensei's Divining Top on top of your library from the battlefield.
Activate Ashnod's Altar by sacrificing Flayer of the Hatebound, adding ({C} magic symbol) ({C} magic symbol) .
When Flayer of the Hatebound dies, its undying ability triggers, returning itself from your graveyard to the battlefield with a +1/+1 counter on it.
When Flayer of the Hatebound enters the battlefield, it triggers, dealing damage equal to its power to any target.
Cast Sensei's Divining Top from the top of your library by paying ({1} magic symbol) and removing the +1/+1 counter from Flayer of the Hatebound.