When Unstoppable Slasher dies, its second ability triggers, returning itself from your graveyard to the battlefield with two stun counters on it.
When Unstoppable Slasher enters the battlefield, Trostani triggers, causing you to gain life equal to Unstoppable Slasher's toughness.
Activate Hex Parasite by paying ({2} magic symbol) and 2 life, removing both stun counters from Unstoppable Slasher and giving Hex Parasite +2/+0 until end of turn.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite lifegain triggers.
Infinite lifegain.
Infinitely powerful Hex Parasite until end of turn.