My understanding is that the game gets harder later, but right now in the early levels it's a total cakewalk. I don't bring any NPCs with me when I go to a dungeon, but they automatically pop in along the way as I go and by the time I hit the final boss, I've got 2 or 3 NPCs in my party, making the fight something I can pretty much just sit back and watch.
Is there any way I can keep the NPCs from joining my party?
If you don't fill the party slots in the Tavern directly with a set NPC ally, then random allies will instead join you as you start each dungeon. Basically you get a choice between either having a fixed set of companions, or random selection if you don't have any fancy.
While in the Tavern, though, you can also choose not to have anyone fill in a slot. On the "Choose Allies" page, you can opt to mark a slot as "Join Off" by pressing Triangle, which will prevent an NPC from just hopping in. Note that this is actually necessary for some quests down the line.
