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

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.