Suggested Location: Sky against Sprinkler and Groundskeeper. (While there are a few places with Arcana high enough for JP/CP, none have enough to make this trial easy and thus aren't worth discussing.)
Side Note: If at all possible DUO or TRIO this trial as anyone doing it that gets a kill shot using the WS will count towards your trial. If everyone attacks their own mob to ensure they get the kill shot without someone killing it with a normal melee blow it is also easier. If you all attack the same mob make sure you coordinate who uses a WS to knock the mob down and who saves their TP to make the killing blow! While doing my WS trial for the WS +10% I infrequently did a Tachi: Gekko at the same time as my friend, killing the mob with the wrong WS.