What's a smart way to play it to earn more achievements? I really have no idea, I went thru the tutorial only to get Conker and was rather underwhelmed by the experience.
While using Premium to fully maximize returns:
Firstly - Grinding the Challenges: Daily Goals, Milestones (going from Tier 1 to 10) that will give the most Credits/XP in the shortest timeframe. You should (easily) aim to make 100-160k Credits, which you can then use to purchase a DLC pack, that has a 30-Day Premium attached with it (I went for Champions Quest Play Bundle). With Premium you can purchase things by just using the Credits earned, at a discount too. However, without it, you'll also need Spark Tokens which are slower/harder to come by (or you'll have to pay for them). So best to go hard while you got it, thats the key. Everything Achievement related can be unlocked for free.
People have created their own levels, which will help grind even faster (example - Kill 250 enemies)
Secondly - By logging in everyday, you'll get 3 Golden tickets (instead of just 1 as a Non-premium user) and a Multiplier (starting from Sunday 1.5x, and climbing to 4.5x the following Saturday) to use for prizes in the Treasure Trove Match game (Extra Credits and Tokens).
Its best to play your tickets on the final day when the Multiplier is at its strongest.
Theres quite alot more info which can be found by searchng for grinding tips.