Buy the Double Spear for $1.99. It can carry you through most of the story by itself with it's high attack power and good range. Without it, you have to do a lot of grinding. You won't be able to do very well in the arena or floating continent, but the other IAP require much more work/money before they're effective because the Large Attack skill is difficult to get.
The Sonic Hammer with 5 Large Lightweight skills is an incredibly useful weapon. It's fairly easy to obtain and upgrade. With 5 Large Lightweights and no armor on, you can delay almost any enemy so that you're never attacked. Just cycle through hitting enemies and they'll rarely attack. Adding 2 [Attack Radius Up] skills and an attribute or rage is pretty useful. The only downside to the Sonic Hammer is that battles can take a long time. Anything less than 5 Large Lightweights (0.9 weight total) and the Sonic Hammer is far less useful.
The only way to get the Large Attack skill is through the Spendthrift Blacksmith, the first Blacksmith you meet that gives you random abilities. You need to upgrade/downgrade until you get the skill you want. Same with Critical skills.
More parts of the map open up once you find and break the stone statues.
If you swipe through shops in Freigold and Ironheart towns, you'll find secret shops.
The first time you meet a lone Black Knight, you'll have to defeat it yourself. It has 90 HP. There's no way you can defeat groups of Black Knights without an upgrade though.
If you find an area that's way too hard and there's no stronger armor available to you, you either need to add a skill to defeat the enemies more easily or keep exploring to find an easier path through.
Steel Helmet + Steel Armor + Iron Bangle: Many enemies will do 0 damage.
Steel Helmet + Crystal Armor + Shadow Necklace: Light enemies, like the Pseudosun, do 0 or 1 damage.
Steel Helmet + Gore Coat + Iron Bangle: Cold enemies like Snowgore and Trunklegs only do 1 damage.