To increase your level fast enough, you can complete story line quests with high xp rewards.
But what I was doing to gain levels was:
1) Find a dungeon with good experience/drop (gold with items) reward (usually humanoids)
2) Find the combination of what mercs I have to use to be able to win this dungeon but with less mercs in use
3) Always I use at least one merc (the one with higher gold/item drop bonuses) it helps for the golds and if something go wrong there is someone else to die instead of you.
4) Repeat until you gain a level.
5) Then decrease the merc usage.
6) Repeat 4) and 5) until the drop and xp become less profitable.
7) Go to step 1) .
