You can get decoy when you get to where you're ultimately supposed to go after going to see the Garif. At least, that's where I got it and a lot of the other magic I was wanting (float and haste anyone?). I got a couple of nice ~70 chains in a place after that xD

Marilith is actually pretty easy to get to, I'm just glad I was able to run away fast enough.

I would try to continue with the storyline, use your summon to take out hard groups that might gang up on you and run from the rest if you need to. There's a couple of hunts that open up after you go to Garif-land that aren't too hard, if you can make it through the plains easily.

Definitely don't try to take on the white mousse ;ooo I tried to go down there and ran into just two monsters that had far more HP than I was ready to get rid of in a normal monster, so I quickeninged my way out of that place and hurried back to the surface.