Normally that seems rather steep, but in a token based deck that shouldn't be too hard to overcome
Marketback Walker is a nice early to mid game play in a counters deck, provided you have a way to sacrifice it later to cash in for a hand refill
If a card like Aetherworks Marvel requires you to tap to use the ability, you must first untap Marvel to use the ability again
Changes You Can Make The most obvious change you can make to this Gruul Landfall deck is to add more protection spells : spells that deal damage to potential blockers, add hexproof to your creatures, or, even better, make them indestructible
You build up your tokens, but need to give stuff to opponents to do so, and as long as you avoid giving them cards that cause them to lose, you'll get plenty of 5-star reviews
For example, the effects of a Heroic Intervention will cease when you activate Obeka, as the card explicitly says, until end of turn