I use a deck with a similar strategy (Leafeon w/ roserade&houndoom prime)
so i've been comparing the two decks
Musharna - PP + sleep = 90 damage
Leafeon - C + pioson/burn/confusion = 50-150 damage
it's an interesting trade off
leafeon can tech flareon for steelix tho :p
RE: LostGar (HGSS-on)
you could consider mime jr and slowking CL
you already have the candies so a 1-0-1 slowking won't slow you much
and it gives you some more options
+3-4 engineers
helps with draw and gets those few energies in the discard steelix can grab :)
and if you think about it life herb is better than potion :p - on average it heals 35
potion only heals 30
RE: Miasma Sun (Leafeon, Espeon P., Blaziken & Roserade)
I've been playing with this deck for a few months now and i'm still not completely satisfied but here's my current list:
pokes
2-2-2 leafeon cl - leafeon lv x
2-2 roserade ul
2-2 sunflora gs
2-2 magmortar sv
1-1 uxie
1 Azelf
trainers
4...