First off when you mouse over skills on your bar it will show what stats scale the skill such as Strength or Dexterity. While moused over the skill you can also hold ALT and it will show you more detailed information about the skill and its scaling. This is helpful for learning the best ways to scale all the different skills in the game since each skill will have different ways of scaling it.
Typically flat added damage scales most skills the best but this isnt always the case. There are also skill that wont even scale from % Damage and scale even more from added flat damage like Judgment. And skills like Warpath only scale 40% effectiveness of Added Flat Damage. Since we do not have tooltips for each skill you kinda just have to use these tooltip information on each skill to help you figure what best scales the skill.
In the case of Shatter Strike it scales from Melee Damage, Cold Damage (Melee), Dexterity (+1 Added Cold Melee per), and Intelligence (4% Cold Damage per) and Intel also Scales 4% Freeze Rate Multiplier. Thats the main stats you want to focus on for best scaling this skill. Something to consider since you have 2 main stats that can scale this skill is what else do you want to scale? for instance what Defenses do you wish to use on this character? Do you want to use Ward and scale Intel so you scale the %Cold damage along with scaling for ward retention or do you wish to scale it through Dex and go Dodge route so you can scale both Melee and Cold % Damage for the skill while also helping with Dodge Scaling?
These are just some of the things you will need to consider for endgame scaling of this skill. This also isnt to mention what other skill you wish to use but you should get the general idea youll want to pick one of these main stats that best synergizes with the skills you plan to use for the best results.