Glint-Horn Buccaneer triggers, dealing 1 damage to each opponent and causing you to gain that much life.
Heliod triggers, putting a +1/+1 counter on Benthic Biomancer.
Repeat from step 2.
This loop transforms a simple +1/+1 counter placement into a devastating chain reaction of card filtering and direct damage. Once Benthic Biomancer receives its initial counter, its triggered ability forces you to draw and discard a card, which in turn commands Glint-Horn Buccaneer to blast your opponents with damage. Because that damage translates into life gained, Heliod, Sun-Crowned's passive blessing instantly places a fresh +1/+1 counter right back onto Benthic Biomancer, fueling the engine to repeat infinitely.