Im not and have never been an MTG judge, but you can sit back and enjoy this summary of how to use the level up mechanic (insert Super Mushroom sound)
Standard also has two other creatures with this exact mana cost: Craterhoof Behemoth and Ghalta, Stampede Tyrant, which are much more proactive threats
You need to be careful with your triggers on this one, as you want to make sure that the Cactuars 10,000 Needles resolves before the Mage Slayers does, otherwise, youre just dealing one measly point of damage
Heck, I wouldnt be surprised if Lathliss (along with the other cool Dragons in this set) cause a couple of minor Dragon-related Commander spikes similar to what happened with Dragons of Tarkir came out
Which card is your favorite
Voja, Jaws of the ConclaveVoja, Jaws of the Conclave is a Wolf himself, and will always draw at least one card