This will need ridiculous and completely ludicrous moves. That have pretty much no effect than making it more dramatic, until it's starts getting completely ridiculous.
Imperial Guard Player:
Throws his models from his box, pinching them, and then flicking them on the table.
Pulls at the table, revealing that his Baneblade was under a hidden panel all along.
Pours his models onto the table, taking up all of his half of deployment. They are all Boyz.
>Playing Apocalypse "no limit" game with MC and friends.
>As many models as you can get on the table, you can field.
>He only has a poultry force. He's "left them at home"
>Suddenly, when MC and friends have moved forward
>TZEENTCH! GRANT ME MY BOON!!!!
Portals appear on the board. Models just keep coming through, almost overwhelming MC and friends. But...something bigger is going to come through...
>"Huh. We're timing our turns. That many models could never be moved fast enough."
>COMPLETE BIOLOGICAL SYNERGY
Starts moving hands too fast to follow. Models look like they're moving on their own.
Also, Ork Player(s) betray Chaos player, because "If these guys don't stand a chance, then a better fight will be against you!"
And MC hallucinates that Emperor is guiding him. In fact, turning into Commissars, Tanks crashing through wall and stuff, is all imagined by the characters. Only Chaos BBEG actually BELIEVES that it's all real. Ork player just follows the Ork Player(s) philosophy.
But, if none of it's real...how did the portal's appear...?