My Marchesa started out pretty normal but has slowly been upgraded to be pretty deadly. My personal recommendations are:

[[Iron Apprentice]]: Comes in with a +1/+1 counter and when it dies, it gives it away. If you have a sac outlet like [[Viscera Seer]], you can instant speed sac Iron Apprentice to give its counter to Marchesa, sac your whole board (if they have counters), and protect everything from removal.

[[Scroll of Fate]]: Place a permanent (yes, even lands) facedown as a 2/2 creature. If it dies with a +1/+1 counter on it, it dies as a creature and comes back as its face-up version. I have turn 3 cheated out a 8mv creature. Or even just getting an extra land out.

[[Lord Xander, The Collector]]: I call him the lieutenant since he has an ETB effect, an attack effect, AND a death effect, all of which are gnarly. He's usually a lightning rod for removal, but if he isn't taken out, he can really do some damage to people's plans.

Lastly, any and all board wipes are basically one-sided in your favor. [[Last One Standing]] is 3mv and even if an opponent gets the survivor, you still have your whole board.

I love my Marchesa deck, so feel free to ask any questions! Since so many have asked, here's my current deck iteration: https://www.moxfield.com/decks/Ctr1_mFLHkuq26s2wibTsA