To determine if Pelt Collector’s first ability triggers when a creature enters the battlefield, use the creature’s power after applying any static abilities (such as that of Trostani Discordant) that modify its power.

To determine if Pelt Collector’s first ability triggers when a creature dies, use its power as it last existed on the battlefield.

If a creature with power less than or equal to Pelt Collector’s power enters the battlefield or dies, Pelt Collector’s first ability doesn’t trigger at all. You can’t try to raise that creature’s power or lower Pelt Collector’s to get a counter.

As Pelt Collector’s first ability resolves, if the entering creature’s power is no longer greater than Pelt Collector’s power, or if Pelt Collector’s power has been raised to greater than or equal to the power of the creature that died, Pelt Collector doesn’t get a +1/+1 counter. Notably, this means that if Pelt Collector is still a 1/1 creature and two 2/2 creatures die, it only gets one +1/+1 counter as the first triggered ability resolves, and it will be too big to get a counter as the second triggered ability resolves.

If the entering creature leaves the battlefield while Pelt Collector’s triggered ability is on the stack, use its power as it last existed on the battlefield to determine whether Pelt Collector gets a +1/+1 counter.

If Pelt Collector’s power becomes less than 0, a creature with 0 power (or with power less than 0 but still greater than Pelt Collector’s power) entering the battlefield or dying will cause its first ability to trigger.

If Pelt Collector is dealt lethal damage at the same time as a larger creature you control, they’re destroyed at the same time. Pelt Collector won’t get an additional +1/+1 from its ability in time to save it.