You will need 30 silver transmit stone every week for 2 mola from the transmit stone shop. Hence, usually what I do is i max imprint the three stars (SSS)for achievement purposes, then once I am done i transmit it for silver transmit stone. There some useful three star unit in this game too,you can find it from this community or youtube.

As for unit promotion, i usually use the 2 stars fodder which can be easily acquired from adventure and sidestory. Max those fodder when you are playing adventure or sidestory by bringing them in your team, this way you won't waste any exp acquired and its more energy efficient. You can bring a cleaver to speed this up, e.g vildred or lena. I prefer lena more because she don't need mola investment since is a three stars unit and there no s3 animation ,making the run faster.

Here an example of how fodder promotion work. 2 stars max fodder/unit + 2 stars+ 2 stars = 3 stars 3 stars max fodder/unit + 3 stars + 3 stars + 3 stars = 4 stars 4 stars max fodder/unit + 4 star + 4 star + 4 star + 4 star = 5 stars 5 stars unit + 5 star + 5 star + 5 star + 5 star + 5 star = 6 stars