[{quoted}](name=Derek the Ferret,realm=NA,application-id=3ErqAdtq,discussion-id=p7smTwTb,comment-id=,timestamp=2015-05-30T09:59:55.557+0000)
The way Riot describes things, the words "skirmishing" and "assassin" should never be used in conjunction. Skirmishers are people who deal sustained damage. Assassins are people who deal high amounts of burst damage. See why those shouldn't be combined? Because you get a guy who deals high amounts of sustained burst damage like Ekko. Then Ekko has a ton of CC and mobility tacked onto his kit.
Riot's definition of "assassin" is mobile, agile and opportunistic. It has nothing to do with "burst", which is actually an attribute of the "mage" class (hence the large number of mage assassins - although currently mage is only used by AP champions, so Zed is an outlier).
Riot's definition of "skirmisher" (aka duelist) is someone who excels in small fights (aka skirmishes). However, skirmisher/duelist isn't actually a class. "Fighters" deal sustained damage and are often good in skirmishes but they aren't synonymous and Ekko isn't a fighter.
Ekko is an assassin (opportunistic and agile) and he is a skirmisher (he's a strong duelist), but he should be classified as an Assassin/Mage not a Assassin/Fighter.
Riot could stand to rename "assassin" to "rogue" to avoid confusions between "assassins" and "champions who can assassinate" (similar to why they renamed carry to marksman) - but as far as Riot's definitions go, Ekko is appropriately defined.