Ignous the Firemonger is one of the three possible bosses for floor 4, and can be considered the hardest of them.
Ignus' room has flat floor, 4 columns, and grated floor on the perimeter.
Immediately after appearing Ignus will spawn multiple Storm Beasts. The attacks are limited to spitting fireballs in several patterns (one being Magma Rifle like spread attack, and other firing a sequence of fireballs following the current player position), but don't let that lower your expectation - each fireball leaves a moderate area burning for a while, meaning that without appropriate damage output you can eventually simply run out of non-burning ground to stand on.
It appears to be intended that you hide from the boss' attacks behind the column and peek out between them to attack, periodically changing columns as the boss (and Storm Beasts) approach the current hideout.