Essence Channeler’s first ability cares whether you’ve lost life this turn, not how your life total has changed. For example, if you’ve gained 2 life and lost 1 life so far this turn, you’ve still lost life.

Once Essence Channeler has been blocked by a creature without flying or reach, losing life to give it flying and vigilance won’t cause it to become unblocked.

Essence Channeler’s second ability triggers just once for each life-gaining event, no matter how much life was gained.

If Essence Channeler is dealt lethal damage at the same time that you gain life, it won’t receive a counter from its second ability in time to save it.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Essence Channeler’s second ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

If you gain an amount of life “for each” of something, that life is gained as one event and Essence Channeler’s second ability triggers only once.

In a Two-Headed Giant game, life gained by your teammate won’t cause Essence Channeler’s second ability to trigger even though it caused your team’s life total to increase.

Essence Channeler’s last ability puts all counters that were on Essence Channeler onto the target creature, not just its +1/+1 counters.

Essence Channeler’s last ability doesn’t cause you to move counters from Essence Channeler onto the target creature. Rather, you put the same number of each kind of counter Essence Channeler had when it died onto the target creature.

In some unusual cases, you may end up putting the appropriate counters on more than one permanent. For example, if you control The Ozolith when Essence Channeler dies, you’ll put the appropriate number of each kind of counter onto both The Ozolith and the target creature.

If Essence Channeler has -1/-1 counters on it when it dies, that ability will include those as well. This may result in the recipient of the counters also dying.

If enough -1/-1 counters are put on Essence Channeler at the same time to make its toughness 0 or less, its last ability will see all of the +1/+1 counters it had when it died as well as the -1/-1 counters it had, and an equal number of each of those types of counters (plus any other applicable counters) will be put onto the target creature.