The Goal:
Agatha is a powerful 6 drop with a considerable downside, she plays the game for you. For those who don't know, Agatha's A.I is essentially non-existent. Agatha plays cards randomly in random locations until she runs out of Energy. The only exception to this is that she ALWAYS plays herself if she has the option to. She doesn't check locations, so yes she can play herself and your other cards into Death's Domain, for example.
Agatha's downside is, essentially, randomness.exe. So our goal is to control that randomness. We have two main ways of doing this, Wave & Lady Sif. Lady Sif discards our Agatha. Wave discounts Agatha down to 4 energy, meaning she will play herself on turn 4.
Agatha does come with a "hidden" upside, she starts in your hand before you draw your opening hand, meaning you have 1 extra card in your hand & 1 card deeper in the deck. This means the odds of you drawing either Wave or Lady Sif by turn 3 are increased!
Card justification:
Domino or Quicksilver
Domino and Quicksilver are "doubled-edged swords" in Agatha lists. On one hand, it gives you something to do in the early game that won't disrupt our ideal turn 3. On the other hand, the odds of drawing one of our key turn 3 cards are reduced. Without QS or Domino, the odds of drawing either Wave or Ghost Rider on or before turn 3 is 81.81%.
If we run QS or Domino, those odds drop to 77.77%. Which is a drop, but it's manageable. I don't think we ever run BOTH. At that point, our draw odds drop to 61.8%/
(Thanks to @VinKelsier for Math, my brain small)
Wave
Wave lets us play Agatha on turn 4.
Lady Sif
Lady Sif gets rid of Agatha and lets us play the game.
Ghost Rider
Ghost Rider resummons our 14 Power Agatha for the low cost of 4 energy.
Shang-Chi & Enchantress
Shang-Chi & Enchantress are powerful tech cards, but more importantly they don't cost 3 or less, in fact they are the perfect cost for this particular deck & have fantastic synergy with Sera.
Drax
Now I know what you are thinking... "Drax?!" Drax is secretly really good in this deck, we are basically only ever playing this card on turn 6 and by that time in the game its decently easy to tell where the opponent will play. So in theory, Drax should always be 8 Power when it matters.
Jubilee
Because we are running Agatha, by turn 6 we only have 2 cards remaining in our deck. So if we are paying attention, we know the two cards we are taking a 50% chance to "play." We can also play Jubilee if we don't have Ghost Rider, Sera, or Taskmaster in hand to try and "reach" one of them. Though, that's certainly not ideal and in that spot its likely a retreat angle.
Typhoid Mary
Typhoid Mary is a lot of power for 4 energy. Additionally we run Enchantress, which can counterbalance her "downside."
Taskmaster
Taskmaster has fantastic synergy with Agatha after being played from Wave's effect. Though to be honest, he doesn't have much synergy outside of that in this deck.
Sera
Sera enables our strategy. We want to play Sera or Taskmaster on 5. If we play Sera, all of our 4 drops become 3 drops which sets up for an impactful turn 6.
Agatha Harkness
Agatha is our primary build around card.