Here are the pronunciations that make the most sense (“ə” is a lazy vowel sound, like the “i” in “pencil” or the “u” in “circus”):
Braixen: BRĀK-sən (sounds like “breaks in”)
Aegislash: Ē-ji-slash (rhymes with “bee kiss cash” (I ran out of ideas for rhymes))
Gourgeist: GÔR-gīst (the “Gour” sounds like “gore”, the “geist” rhymes with “heist”)
Yveltal: I usually say ē-VEL-təl but it could be Ē-vil-täl (like “evil tall”)
Trevenant: TRE-və-nənt (rhymes with “revenant” or “heaven meant”)