basically have every unit in its section ready to go except a spell caster - in this case infester - send them to a section you want extra units in and spawn as many infested terrans as you want, then have the infester go to whichever section you want him in later and as soon as he gets there start the challenge - doing this will fill the infester mana back to full
the extra infested terrans will be able to eat some of the fire so you can get your units in place and with some burrow micro it is a lot easier to not lose any units
*here's a vid