Pouring a slab that size really isn't a DIY project unless you have the tools and a few friends to help. You can certainly build a ground level platform out of pressure treated lumber. Unless you have very well draining soil I might suggest excavating some of the top soil and putting down a layer of crushed stone.
If it's only you, concrete is definitely out. That leaves a wooden platform.
"Easy" probably isn't going to happen. There are lots of projects that two people can complete in less than half the time of one person working alone and this is one of them. A Chris said, you'll have to stick frame it and getting everything level is going to be challenge. Years ago I did a deck that was at ground level. It took a while with two of us who knew what we were doing.
Maybe you want to get a couple of estimates for someone to pour a slab...