Miang Moonspell is found in the top right corner of Mt. Moonspell in a Coffin inside of a building. Then purchase her for use in the character select screen.
This achievement requires you to survive until there are 15 minutes on the clock. The easiest/fastest way to do this is to do the "Il Molise" (which has unmoving enemies) on hurry, to reduce the time required to 7 and a half minutes instead of 15. At the same time, you should evolve the Silver Wind weapon by combining it with a max level Pummarola to unlock Menya Moonspell and another achievement.
TLDR:
Character: Miang Moonspell
Modifiers: Hurry (and any others)
Objectives: Survive until 15 minutes, evolving the Silver Wind with max level Pummarola, then quit from the menu
I recommend you use the same strategy for three other achievements:
I recommend that you use a different map for the characters that require 100,000 kills in one map, as it would be extremely slow here.