You could conceivably spend levels 21-30 IN Act 1 because the end dungeon is fairly difficult, but you will yearn to go to Act 2 because level ups will become painfully slow and, because you are not dying, you will build up a lot of ember chips to merge up.
After my level 23 or so Embermage bit the dust, I ran Elite Hardcore with a Berserker and used THIS build strategy. There are tons of solid builds out their on the interwebtubes to look at for all classes. You better know your build and how it’s going to both deal out and survive damage. That said, here are ten more tips if you do choose to keep going past level 20. Act 2 comes down on you so hard that you will need aircrash coroners to piece you back together again. I want to tell you straight away: you will not make it through Act 2 Elite hardcore. So you made it to level 21 in Torchlight Hardcore Elite aye? Think it was tough? Think it was a slough and many of your characters died even with my other list of tips as reference? Well Act 1 is fucking CHEESE CAKE compared to Act 2 Elite.