Decks

A fairly accessible budget deck for beginners, using only two mythics: Desertification Engine and Armageddon Angel. If you lack these mythics, I would sub in 2 Serendipity Ifreets and play more aggressively. Ifreet is good on turn 3 and can help search for Dune Cat / Dust Devil to get the game rolling.

The gameplan is to begin by generating 1 or 2 deserts from Dune Cat or Dust Devil. From there, you make use of either Sand Scout or Earth Striker to kill enemy minions for free and feed your coliseum. Sand Scout doesn't take retaliatory damage when attacking while occupying a desert, while Earth Striker has alpha strike (and can also draw a card if you play it on a desert). Singing Stone is a cheap 1-mana artifact that gives all minions on enchantments +1/+1. You can further buff these minions as well with Dark Passenger or Vengeful Heirloom.

In the mid-game, you have Tailroot Wurm or Terra Forma, which are both absolutely huge for their mana cost when played on enchantments. Mardykhor is good for pinging annoying low-health minions like Wings of Abaddon or Mothmara. It has the added benefit that when it dies on a desert, it can deal 3 damage to anything.

Finally, for late game, you have Hyperborean and Sandscale Wurm, with Resupply Caravan or Allfather's Horn for extra burst.

This deck does have bad matchups. Against decks that play a lot of spell or ping removal, such as Detained/Deported, Anhelli, Ignition, Crimson Pact, Plague Maidens and so on, it doesn't have a good way of coming back from behind. Coliseum in general is the kind of path where you want to be constantly ahead. Once you fall behind, it's hard to recover. (Angel is probably the only recovery option that can completely swing a game around.)