MCX - Crackthorn beast is OP
A pretty standard crackthorn list that i used as my first deck for the monthly cup. It won me quite a lot of games because it counters well blue, you also have good matchups against red and can make your way again yellow, it is well countered by green but there wasn't that much in the cup.
Usually build to one side and make your way to the opponent's orb (try to get a moutain next to your opponent's orb to get value out of your axe grinders).
Against blue, i like to build defensively to both sides to prevent them to collect on my wells and reduce their colosses.
You have great mobility tools thanks to sagami and horsemaster (be carefull on your forest placement to have a maximum of possibilities), and great aoe power too.
Mulligan: keep a T1 creature (axe grinder, sagami warrior, wood elemental), seifer and groundshaker, you can keep a seifer wrath or a flame burst in you have other creatures in hand and are playing against blue, red or yellow.
Concerning crakthorn beast, try to think how you can have maximum value from it (for example, it's sometimes better to play a groundshaker or a blast before the beast, to ensure max damage to your opponent's creatures). The dash effect can pressure the orb even if you didn't manage to get aggressive lands.