Unlock scathach perm?

You could finish part 2 of the story coming in the 6th. Part 2 is the second phase of the event that adds new quests, banners and mats. I'm not really familiar, but I heard you just need to finish the main quests of part 2 (if I'm wrong please correct) and you get her permanently
http://fategrandorder.wikia.com/wiki/FGO_2016_Summer_Event/Main_Quest_Part_II
Also please refer to her Scathach assassin or summer Scathach since there are two in NA with her lancer form and 3 in JP with her caster form.

If you're talking about the current event: You unlock the swimsuit Scathach as a "temporary" servant after completing part one of the event's story, which is going on right now. You then buy all of her Ascension materials, the "Bell Ringing Branches" in the event shop using the materials you can collect from the event missions and whatnot. Then when you complete part two of the event story, which unlocks in about a week or so, she will join your party permanently. Hope this helps, I can explain in more detail if needed.