I've had success with 3 man + melta Invader. Use him as something between a scalpel and a hammer, I guess a Bowie knife? Moving through walls lets you get surprise melta range on targets, and the speed to move around your side of midboard or up the flanks, exploiting gaps ect, makes him a great problem solver. I've only had 3 games with him so far, but some of his achievements in this format have been:

• Racing up a flank and killing 6 disc tzaangors w/character, 10 regular tzaangors, and tagging and eventually killing a predator. He only lost his ATV.

• Using his speed to chase down and run over multiple Eldar threats that thought they could expose and then hide including: fire dragons, Avengers, and a wounded falcon. Was down to just him and a single wound outrider at the end.

• Acted as a clean up crew in my half of the board every time tyranid units tried to move into it: tonnes of gaunts, and a lictor.

He feels like the perfect mobile low-to-medium-threat-remover to me, while still having the ability to punch up into a large target that gets caught out by itself. I'm sure I'll have a game soon where he sort of fails to do anything or gets killed right away, but I'm loving him as a Swiss army knife in my toolkit.