This deck has a pretty sleek curve and you have good potential to curve out very powerfully with something like:
Turn 1: Thoughtseize or Inquisition - perfect information and remove their best card
Turn 2: Leonin Arbiter - at best block or slow down a fetch, at worst prepare for Maralen.
Turn 3: Maralen of the Mornsong - Playing her turn 3 after leonin is very powerful if your opponent can't break out that exact turn, but it's not always advisable and you might want to play Aven here or bitterblossom + blow up a land with Ghost Quarter.
Turn 4: Madcap Experiment - This might close the game on it's own depending the matchup, and at worst it's probably a must answer threat.
Ok so what about real games?
Well one of the most important decisions is when and with how much setup you want to play Maralen in the given matchup, this isn't easy and takes some learning. If you play her with insufficient setup you may get blown out by the card your opponent tutors, but quite a lot of the time she can be safely played with just 1 supporting card as you'll get enough value and pressure from that play. It's also important to remember that in the final stages of the game she can be played with no support as a straight up 3 to the face when that's enough to secure a win, like you're opponent is at 6 life and you're holding a bolt, or if you're opponent is at 3 or less obviously. If you're up against a deck that folds to Emperion game 1, for example burn has nothing maindeck to turor up against it, you only need 1 tutoring or a window to play madcap to win the game, and in that situation it might be correct to just throw Maralen at them to give them something to deal with.
You can also play without Maralen. You have a good midrange stack of cards, hand disruption, removal, token generators and a powerful finisher in Emperion. There is no need to obsess over setting up for Maralen if you're playing against someone that is better dealt with through hand disruption and removal and grinding it out with bitterblossom and so on. If you're winning against a combo deck for example and 1 Maralen tutoring could potentially help your opponent turn the corner.
One of the trickiest interactions to judge and understand is how Leonin and Ghost Quarters works with Maralen, it's very easy to underestimate it and not play Maralen when you should just because you're afraid of the card your opponent will fetch. The thing is, especially early game the cost to fetch that card often limits it either to removal for Maralen (in which case you netted 3 damage), or a card that can't be played that turn because there isn't enough mana left, or a land to prepare for the next fetch. Now what you're probably doing on your turn is paying for arbiter and fetching a Ghost Quarter to blow up a land. If they fetched a land they made no progress, if they fetched a card they couldn't play they still won't be able to, if they ever get to 1 land or you play a second Arbiter they are hardlocked. After playing the deck some time and trying more and more to "just go for it" I started to realize how often this happens and how much sense it actually makes to go for this lock already on turn 3. It's generally better the earlier you do it, and I've won most games where I did even in the cases where they broke the lock, as trading up in resources is also winning.
Mulligans are worth a mention here as well I suppose, and the most general rule for this deck, apart from the usual common sense judgement about land count and having decent early plays, is to mulligan to 6 most hands with a Platinum Emperion and all hands with 2, I wouldn't mulligan an otherwise reasonable hand to 5 because of 1 emperion though, and it makes more sense to keep 1 emperion if you also drew a collective brutality and are expecting to want to escalate it. Turn 1 thoughtseize is great, seeing Maralen + Aven or Arbiter is great too. Leonin and Ghost quarters is usually good news, especially on the play, and bitterblossom is a great turn 2 play. In short there are many different kinds of keepable hands.
Try to close games from any position of advantage. Especially if you get Emperion out and they don't have an immediate answer, it's not a hard lock and most modern decks have some card in the main that can kill it, so in order to capitalize on it you need to do damage. Tutor up bolts and throw at face, remove chump blockers to push emperion through and things like that. Don't worry about complete locks and board dominance, just win.
