Featherweight DA Ranking for Heavyweight Keywords

Hi SEOs!

We are a featherweight local window & door company (DA12) looking to rank for heavyweight keywords like "replacement window". Since launching our website 7 months ago, we got to 4th place for the keyword "buy windows online" meaning we are doing something right.

If you had to use only 1 tack tick to gain PA weight "fast", what would it be?

If your suggestion is backlinks, then what type of backlinks?