If Hallar leaves the battlefield after its last ability has triggered but before it resolves, you don't put a +1/+1 counter on anything as the ability resolves, but you do use the number of +1/+1 counters that were on Hallar before it left the battlefield to determine how much damage it deals to each opponent.

In a Two-Headed Giant game, Hallar's last ability causes the opposing team to lose 2 life for each +1/+1 counter on it.

Hallar's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.