Elementalist works. I've got one. Lightning is my favorite. The Ele does take a penalty in Hard Mode. But the decrease in damage is not critical. With a good 7 hero party my Lightning Ele does fine in Hard Mode, and of course normal mode is a cakewalk unless I get snowed by aggro.
Hard Mode is available only after you have completed the final mission in at least one campaign. The Nightfall campaign is a great place to start. The starting island is very beautiful, mostly, and cake take you to level 18.5 approximately. At that point you do a mission that takes you to the mainland where Nightfall really gets rolling.
I personally liked the story in Nightfall a great deal. It is a good story. You get a number of heroes in Nightfall and it is fun to set them up and configure them as you get new skills along the way.
Each campaign has a story, but the three stories are somewhat independent. Enough so that the final expansion Eye of the North is necessary to tie them all together. You do not have to play the campaigns in order.
Do try and find a good guild. That will help a lot.
http://www.guildwiki.org/Main_Page has lots of good information as you start out and progress through the game.