This achievement is complicated by the fact that the [Spring Flowers] have a five minute cooldown, but it is not impossible.
Check by the opposite faction's bank or entrance to their quarter in Dalaran. Ensure that your target is not already wearing bunny ears (Check their buffs! Look for a carrot icon), though this won't trigger a cooldown.
If you don't mind a little PvP, you can get this achievement very quickly. Just queue for Arena Skirmish matches. The cool-down is reset for your [Spring Flowers] each match, because it's shorter than 10 min.
The fact that the females have to be at least Level 18 is an obvious pun on 18 being the legal age for, ahem, all sorts of things.
To make sure that the target IS a girl (Hey, the helmets are distractive!) you can use this macro:
/run local x=UnitSex("target");local g={"n is not a player!", " boy!", " girl!"} n=GetUnitName("target");if IsShiftKeyDown()then SendChatMessage(n .." is a" ..g[x],"SAY")
;else print(n .." is a" ..g[x]);end
Addons on curse.com that make this achievement much easier.