If a creature is attacking a planeswalker, that planeswalker's controller is the defending player. If a creature is attacking a battle, that battle's protector is the defending player.

If, during your declare attackers step, Manticore Eternal is tapped or is affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having it attack, you aren't forced to pay that cost, so it doesn't have to attack in that case either.

If multiple creatures block a creature with afflict, afflict triggers only once.

Afflict causes the defending player to lose life; it's not damage or combat damage.

Afflict resolves before combat damage is dealt. If this loss of life brings a player to 0 life or less, that player loses the game immediately. A blocking creature with lifelink won't deal combat damage in time to save that player.