Nirkana Assassin — ruling 1

The ability triggers just once for each life-gaining event, whether it’s 1 life from Drana’s Emissary or 7 life from Nissa’s Renewal.

Nirkana Assassin — ruling 2

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

Nirkana Assassin — ruling 3

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

Nirkana Assassin : règles, fonctionnement et interactions (MTG)

Nirkana Assassin is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever you gain life, this creature gains deathtouch until end of turn. (Any amount of damage it deals to a creature is enough to destroy it.)

How it works : Nirkana Assassin

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2015· Wizards of the Coast
The ability triggers just once for each life-gaining event, whether it’s 1 life from Drana’s Emissary or 7 life from Nissa’s Renewal.
No other card is named in this ruling.
#2August 25, 2015· Wizards of the Coast
A creature with lifelink dealing combat damage is a single life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, the ability will trigger twice. However, if a single creature with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.
No other card is named in this ruling.
#3August 25, 2015· Wizards of the Coast
In a Two-Headed Giant game, life gained by your teammate won’t cause the ability to trigger, even though it causes your team’s life total to increase.
No other card is named in this ruling.