As for specific reasoning for the creature choice, I’ll offer the following:
Minature and Denn Wildborn: She’s great for triggering cascades with her generating 1 green/red mana per turn. Also if you hit her activate, this can help you clear the opposition without losing 100% of your damage, thanks to trample. Overall this helps with the speed and the longevity before needing to drink a potion.
Akoum Hellkite: Solid damage to cost ratio and also clears enemies when you get landfalls.
Avaricious Dragon: A very high priority to bring out when facing any PW who doesn’t have red as a color. This is because it has a very good damage to cost ratio as well as cascade potential every turn.
Jovi En, Ruin Diver: A cheap way to double your card draws, giving you faster access to the better creatures as well as the highly desired Mirrorpool.
Oblivion Sower: Another solid creature to bring out, preferably early on. I only suggest you prioritize this creature when the opposition is not red.
Scab-Clan Berserker: Haste is always a bonus when it comes to speed, if you manage to hit the enemy instead of a creature, you’ll get a bonus +3/+3, this would be my target to replace with Firebird if I had it. I usually only give this card higher priority when I know I can hit the PW and not an enemy.
Tyrant of Valakut: This creature may seem a bit over costed, but it has 3 very powerful things going on. One, it is your best survive ability tool, often clearing the enemies board, allowing you to preserve health. Two, it offers 6 bonus damage to the plains walker immediately. Three, it receives the highest bonus from Mirrorpool thanks to the powerful starting stats.
Volcanic Rambler: Probably my lowest pick for bringing out late game, but if it’s in your starting hand, it should have the highest priority because you can get it out and trigger some very powerful cascades, potentially filling your entire hand in 1 turn.
Additional Info: Because a lot of creatures have situational usefulness, (eg. Oblivion Sower/Avaricious Dragon vs non red, Jovi En when low on cards, Tyrant when the enemy has 2-3 creatures, Scab-Clan Berserker when no creatures are present) it is always good to have several options available.