So I have beaten Starcraft 2 on Normal, Hard, and Brutal yet it says I am at 75%, do I really have to play it again on easy? 
If someone could tell me that would be great! 
Thanks

Achievements.
 All of the branching missions.

You need to find all the areas in the second castle to get 200.6%. 
 
Wait, what? 

...lurk harder.  You missed something.

There's a secret level in the stage where you help out Odin or whatever. You have to play it when it comes up in the story, you can't go back to the mission and do it.

" There's a secret level in the stage where you help out Odin or whatever. You have to play it when it comes up in the story, you can't go back to the mission and do it. "

OP you can post a link to your battle.net profile and then we can easily tell you what you are missing.

How hard is it to click on the achievements tab and look?  Literally takes 5 seconds.
 
That said, did you beat the entire game in under 8 hours yet?  Did you max out every upgrade section at least once during your various playthroughs?

% is based on achievements. I'm only missing the raid night achievement and have 99%.

Play multiplayer.

There are also a few hidden achievements for doing specific things, which are filed under "Feats of Strength" which are also campaign related.  For example: "The Scenic Route" on the Devil's playground where you have to kill all zerg structures.  It isn't listed until you do it.