@Mr Stoat After numerous attempts last night the group I was with finally got to the final wave. Regarding Wave 19 the load out that aced it for us was 3 Assaults & 1 Tac - 2 assaults with Thunderhammers, 1 with Chain Axe and the Tac sporting dual wield storm bolter & melta gun. Get as many people on 1 of the points & cap that don't try splitting your forces between the two. Use the Assaults to pick out shootaz and grenade the hell out of the large groups as they converge on the capture point. Hit & run with the Assaults, ground pounding large groups but making sure you've got enough fuel to leap out almost straight away.
From my perspective as an Assault the loadout that worked really well was Chain Axe, Axemans Zeal and Sure Strike. I normally go for Death From Above, but the radius and additional stuns of the Sure Strike (it takes 2 Y button presses to launch the massive ground slam) really ties up the hordes of Orks and Imperial Guard. It also stuns groups of Space Marines allowing your team mates to whittle them down.
How on Earth you get past the bonus wave is something I'll need to read up on, it's mental!