Brallin triggers, dealing 1 damage to each opponent and causing you to gain that much life.
Benthic Biomancer triggers, putting a +1/+1 counter on it.
Repeat from step 2.
This clever loop starts by putting a +1/+1 counter on Benthic Biomancer, whose triggered ability immediately forces you to draw and discard a card. That discard instantly rouses Brallin, Skyshark Rider, which deals 1 damage to each opponent and implicitly fuels Light of Promise whenever your lifegain links up with it. Because Light of Promise responds to that life gain by placing another +1/+1 counter back onto Benthic Biomancer, the Biomancer's counter-triggered ability fires off again to restart the whole cycle.