You can get by without a guide – we have every year we went. The Portes du Soleil map shows the main trails, and the lifts that are open. Having said that, we went off piste exploring a few times and ended up on some dismal trails!
Definitely get a lift pass for the duration of the trip, it allows access to such a huge area, you can cover serious ground with it.
We’ve had good days by taking a car across to Chatel/Super Chatel and Morgins and riding there, rather than spending a few hours lifting and riding to there.
Other advice – take mud tyres. And waterproofs. It’s quite common for it to rain every day for a week, and if you want to do any riding in this case you’ll want the tyres.