@Dhu-cerbin if you or anybody else with same problem, still need help, I can tell how I did it and make some hints.
First of all, deck from @Imightbuy looks like solid base. However, when you make "play 40 card deck", you generally do not want there anything else than just units. Artefacts and Special cards do not "repeat itself" by playing another card of same provisions. That means at example if you do not have any relevant combo, Runestones or Hen Gaidth Sword (which will not repeat themselves) are as good as any usual unit (which will repeat itself and will thin your deck to get all your important cards).
I checked my collection and looked for "play" and "summon" keywords. Then I checked also for cards, which will make me draw another card, so my turn will be prolonged by playing extra cards.
Then I tried to compose deck the way, which will make sure, that my important cards will be "guaranteed to play". I mean at example since Zoltan: Scoundrel will make you play 2 cards and then repeats itself for another 2 cards (playing cards of same provision), I made sure to have important cards of their provision to be only ones with that provision. In other words, card, which is making you to play more cards should have not same provision cost as any other card in your deck, which do not make you play extra cards.
I made deck based on those principles and I managed to get that quest on the first try. I made Emhyr var Emreis deck (leader himself will allow to play useful card extra time) and my important cards to get that quest were ...
Isbel of Hagge
Roderick of Dun Tynne
Respect to those who did it with a normal deck, here's how you get it done though.
Make a NG deck and spam as many spies as you can. Use cards to replay said spies to fill the rest. Your goal is to spam as many spies as you can during the 3 rounds. Took me 2 tries with such a deck and I didn't make a huge effort. I got lucky and had my opponent playing spies as well so I had access to a few of his which was luck.
Yeah, with NG spies its really easy to do it, tried a deck i built, wasnt optimized or anything (i've had little experience with this seasonal mode) and im pretty sure i've done that condition several times without trying.
But im not filling that reward tree, i dont want no elven trinkets, im saving RP for next season or maybe the next (havent checked what they will be yet)
(EDIT) it's a bit cheap using NG in this seasonal mode - only one more powerful is demavend charges deck probably - but it feels good to be oppressive for once, and its the opponent that needs to be careful not to lose CA, and i can go for 2-0 most times.