Yeah, you would need to grind your head off, unless you buy expansion packs.
You can go to the higher lvl areas and grind as well, but you can't do any of the quests there.
But there's a way that you can get quite a few credits for the store: perks. Many of them are for killing X amount of monsters, and you gain a title + CS credits. So it is possible to buy something from the CS without paying a dime.
