Guns are your best friend for this achievement. They are the cheapest towers and so have the lowest impact on your score, while still being quite powerful (especially when fully upgraded). In levels where it is available, use the Boost upgrade "Score Boost" as this will give you that little bit extra which might tip you over to silver. While working towards this achievement, you should also be trying to work towards:
To complete the above achievement, you need gold medals in every level, not just silver.
Note that this doesn't seem to include the prologue. I have got both this and the Alien Tears achievement and I have a bronze medal for Prologue, but gold for everything else.