Shatter might be hard to work with, I don't think a concentrate that hard will turn into a proper mix. Definitely use PG, or some non-flavored terpenes (what I prefer to use)

You can either mix the terpenes from mass terpenes into the non-flavored ones.

The thing with Massterpenes terps are that they're used widely different, instead of using heat you just let sit. Because of this it doesn't mix well into hard concentrates. But another brands terpenes use heat to mix.

What I do is first mix the concentrate with TreesX non flavored terpenes but any other brand will work. Lower glass vial or jar into boiling water (don't let it go past 200F).

Once it's mixed properly mixed together, add Massterpenes and then let sit.