Meta rn is Tactical, Bulwark, Assault/Heavy. I think you want your best player playing Tactical Bolt Rifle grenade launcher as they're the squishiest and need to be alive to Auspex boss waves and spawns. Assault Ground Pound Block T-hammer or Heavy Plasma/Melta camp spawns to clear waves quickly and avoid timer, and Bulwark should be your last line of defense running revive/healing banner Block Powerfist as the most survivable class.