Honestly if you want to enjoy the game, get it for on computer... The DLC worth it, I got the full game on sale for 12,99$. If you want to play legit, don't play local, play ranked games. (Less mods in ranked).
First you should try to beat everything on Med as you did, with 2 chars, I would say squire and monk,(if you dont have a summoner or EV coming in the DLC). Then do the same insane, beat the game again, its not really hard. Before trying Nightmare, farm some gear in insane hardcore survivals. The Throne Room, is one of the easiest map to build a surv. NEVER do a surv without Hardcore On... without hardcore you wont find anything good... You can also play with people on in public games, some players as me, like to help other players and can lvl ur chars from lvl 1 to 74 in like 25-30 min...
Then farm again and again until you got a full myth matching set before trying Nighmare.
Some players are doing kings game runs to get the reward at the end, in these games you get good exp (20M) and about 200M mana each time
An M2K worth 60-80billions mana and a cube worth 160-180 billions (people trade 3 M2k for 1 cube)
people online are looking for Myth, Trans, Sup or Ult gear. for lvl 74,78, 83 or 90+ you can get these kind of gear from Nightmare. You can get a few Myth from insane surv after wave 17.
this can help you:
http://dungeondefenders.wikia.com/wiki/Dungeon_Defenders_Wiki