Level-Up cards must be played onto the corresponding Pokmon like evolution cards (though, in one rule change, it must be the active Pokmon), allowing for a variant of the Pokmon with greater HP, damage and additional attacks - as well as being able to use the moves of the card from which they levelled-up, unlike evolutions
A good aggro card has a nice ratio of cost to size, such as 3 mana for a 3/3 creature, or even something pushed a bit further like a 3/2 creature for 2 mana
When it comes to valuable cards, the most important thing is condition
#3: Roxanne, Starfall Savant Roxanne is an incredible value Commander in Outlaws of Thunder Junction, doubling your mana production and dropping new mana rocks onto the battlefield for as long as she sticks around
You do have to attack with the Legendary Squirrel every turn, but the Indestructible clause offsets this issue
Thanks so much for reading