Early Rounds: Activate Goblin (3) Synergy effect with the Goblin pieces that cost least Gold to win a winning streak. During this period, you can have 2 choices: Stop Buying Exp to keep yourself at a low level (so that you can have a higher chance of getting Goblin pieces that only cost 1 Gold each from the Store), and spend all your Gold to get as many Goblin pieces as possible. This is tơ rank up all your Goblin pieces to 3 stars. If your enemies have reached Lv.7 or even higher levels, Buy Exp with your Gold until Lv.6 has been reached. After that, save 50 Gold to earn Interest every round. The other choice would be, Buy Exp to unlock more slots at Round 5 and Round 9, to earn a winning streak by taking advantages over the number of your pieces, and save to up 50 Gold with the winning streak rewards.
Mid-rounds: If you have more than 50 Gold, Buy Exp with the exceeding part to reach Lv.8. During this period, only purchase the pieces you really need in the Store and activate Goblin (3) and Assassin (3) Synergy effects. Based on that, you can adJjust your Formation according to the enemy pieces.
Later Rounds: When you can deploy 8 pieces on board, keep 50 Gold at hand while _refreshing your Store with the exceeding part for Devastator. Once you have it, deploy your Goblin pieces on board to activate Goblin (6) Synergy effect. Or you can unlock the 9th slot to increase the chance of getting Legendary pieces, and in that way, you can have more chance to get Devastator.