To achieve this, you need to win a game with 5 or fewer cards in your deck. This 5 count includes any infests cards, constructs and heroes. You start the game at 10 cards, so the plan is to at least remove 5 cards and swap out the others with more powerful cards. I came close at achieving this on a 1 vs 1 game a couple of times, I chose to do a 4 player game, which it turned out to be easier to do, I got it on the first try.
The most beneficial cards are
Deranged Aspirant, Venom Princess, Thunder Templar, Verdant Elemental and Heavy Infantry
One thing to note, is to keep a ditch card with you till the end, in case the computer decided to infect your deck with a card, which has happened in a previous attempt. Try to take out as many cultists to gain to honor, killing monsters in the center row is very beneficial.
You can view my video to help you out
Thanks
Killarpong