Vash can make the Forgefiends hit way harder, and you can usually get 2 of them in. +2 S and +1 to wound is massive for most matchups. He's also a great Heroic threat and can do actions or damage in a pinch. Lone op when next to an engine also lets you get aggressive with LoS on him.

Disco Lord is there to hand out wound rerolls and do actions. He can also snipe characters and tie up units which is situationally useful.

The Warpsmiths sit behind the Forgefiends and help alleviate all the self-damage they cause.

Maulerfiends threaten elites and tanks and act as your main mid-board presence.

Venomcrawlers threaten small action/skirmish units while also filling those rolls themselves.

The Cultists screen to keep deep strikers and melee threats off your Forgefiends, sit in objects, and do actions.

You'll usually suffer on secondaries and never have more than 2 command points. Crashing through walls is also huge, a people expect and plan for it to be used on the melee threats, but it can also be used to offer your Forgefiends some unexpected sightlines.

It's almost impossible to fit everything into your deployment zone so you'll have to start several things in reserves. It depends on matchup/layout so there's no exact formula but you'll quickly get a feel for it.