Mysterious Fossil as a Retreat Aid
    I know it sounds silly, but a lot of people don't want to take the risk
when their opponent puts out a Mysterious Fossil.  All of those thoughts ("Is
it a Kabuto?  Is it an Omanyte?  Oh dear golly is it an Aerodactyl??!?!??")
make them panic, and they use that Gust of Wind to bring it out and beat it
up before it can evolve.  Clefairy doll doesn't make players worry nearly as
much as Mysterious Fossil, so if you put one down and they gust it out, you
can choose another pokemon from your bench to bring out next.
    Of  course, this isn't foolproof.  If your Chansey has 10 HP left and you
plop down a Mysterious Fossil, of course they won't gust it out.  They'll
take out Chansey!  And then they'll cross their fingers and pray that you
didn't put an Aerodactyl in your deck anywhere.
    Since opponents panic, if you want to actually evolve it, it's a good
idea to put two fossils down at once, and make sure that the enemy doesn't
have a bench-hitter anywhere.  My strategy is to have 2 mysterious fossils
and 2 fossil evolutions in my hand before putting one down, but that's the
"perfect world" strategy.  Sometimes a solitary Fossil survives and sometimes
both survive the first turn on the bench.  Fortunately most bench hitters
need two turns to evolve, but that means you need to be extra careful and
make sure they don't have a potential bench hitter on their bench.  I think
Defender can help your fossil survive if you're really worried about it, but
even the best strategy is not foolproof!
        Rainy the Furry Blue Mock-Dragon