there is an excel sheet here posted on this forum to keep a track of your progress and planning. Use that. Amazing tool and works offline.
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As far as defenseive buildings go, I have always been a believer in getting your splash damage outta the way first... Mortars and WT's.
Another person mentioned Lab, another big item, hogs to L4 and subsequently, the heal.
Max out completely before upgrading to th9 saves in the long run! Enjoy th8 as it was a favorite of mine!
BACON!!