Mono Primal (Top 20)

This is the deck I used to get from diamond 3 to master this month (31:10). Its very similar to Krullysian in its play-stile (play cheap threats and protect them/get them back for even cheaper when they get killed). It gives up some good threats by cutting T (Wump & Mizo, Master Conjurer), but with the Cylix nerf I really prefer the more pleasant powerbase of MonoP. You also get some good threats as replacement (Prism Golem, Serpent) and you can run Broken Contract (I am still not 100% sure, if 4 are correct or if you cut it down to 2, because they can sometimes mess you up early, but its pretty easy to get the 4/1 with this deck and that free unit often helps to put on pressure. You can also use Know when to hold em to search for Contract while you have a unit in void).
In your mulligan, if we assume that you dont know what you are playing against, you are looking for a good mix of threats (Jarral, Whispering Wind, Prism Golem, Serpent) and interaction (Maveloft, Permafrost, Dazzle), but with a higher focus on threats (you can keep a hand full of threats, but I wouldnt keep a hand with just interaction). You usually want 3 power (Etchings count as power, but can sometimes be awkward for your curve if you already have a Symbol, because then you cant curve 2 drop into 3 drop with a 3 power hand, so keep that in mind). The most powerful start the deck can have is Whispering Wind on turn 2 into attack on 3 to discard a 3 drop, which draws Krull and revives the 3 drop. But just curving a Jarral into Golem is fine too.
What I really like about the deck so far is that amost every common matchup feels open, meaning that the games are decided by player skill (and draw luck) rather than just the deck. The only exception are Sling of the Chi decks. They are basically always decided by the Sling player drawing Sling early or not (if they play it in the first 3 or 4 turns you will almost always lose, if they dont draw it you will almost always win. But thats just a fundamental design problem with Sling).
The Soldiers matchup can be tough if they drop Genetor Dovid I early, but so far I have a pretty good winrate against it. Permafrost is a good answer for buffed Shock Troops and sometimes Dovid, Overgrowth kills most of their stuff and Maveloft is really good, especially with Prism Golem. Vengeful flight can help to close out that matchup, but only go for it if you are sure that they dont hold up Argo's Technique.
Praxis Sentinels is pretty open aswell, and in that matchup its really important to know when you are the beatdown or control (when is it better to be aggressive or controlling). The buff to Opum's Technique is rough because it kills Prism Golem now, but you have pretty good answers for their threats too (mostly Perma and Huntress). Going wide with your threats (Sentinels cant punish you for that) and keeping high attack Sentinels of the board/stunned (to prevent Bond and Opum, the Gemblazer) is usally a good strategy.
Against bigger midrange decks (Xenan, Argenport, Feln) you want to pressure early and keep up removal for their bigger threats (Moonstone Vanguard can be hard to kill sometimes, you often need the market Polymorph for that). Exploit is usually not a big problem for this deck and sometimes even benefits you because it makes Contract active and gives you Krull targets (I dont think I have to explain Krull anymore, but one neat trick I did a few times to make use of a Krull in hand was that I plundered her into a shadow sigil and then plundered that again for a Trove). In the mid to lategame you optimally have a few good threats and can hold protection/revival spells open. Vengeful Flight usually really helps to get damage through (and attacks with Jarral to draw cards) when the board is clogged.
Against the even more controlly decks like Combrei and TJS you basically want to do the same, but focus even more on early pressure. Combrei isthe easier matchup because they have way less early interaction and as long as you hold up a negate for Equalize once you have a good board and they have 3 justice influence, you should be in a good position. TJS is probably one of the hardest matchups, so its sometimes worth to go all in early and hope that they dont have Know Thy Enemy or Equalize to punish you. But if you have a good hand you can still win with the strategy described above.
Good luck and have fun!