I'm a few hours into the game, and I read through the achievement list. Do you get a S-rank in this game by basically finishing the game including sidequests?

Yup, pretty much. I had to grind a bit near the end to get to level 20 but otherwise it's an easy S-rank.

And probably a little bit of grinding at the end.  But yes.  Welcome to one of the easiest S-ranks out there.

Yeah do everything and then grind a little to get the last level. Pretty easy S-Rank.
 
Here's some advice on speeding up the grinding at the end. Set the difficulty to hard and then use the outhouses to teleport back and forth between the Unicorn Meadow and the lord's castle bridge. Clear out one area and then go back to the other and it should be repopulated. If at some point they stop respawning, quit to the main menu and then continue. You should be able to clear out the unicorns pretty quickly by just attracting their attention to get all of them to charge you, throwing down the invulnerability bubble and then using the spinning blade justice move to kill them all.

And "little grinding" isn't a euphemism for 4 hours.  I think it took me 20 minutes to go from close to 19 to 20 after finishing the game. 

I'm stuck on the orphan quest if anyone could help me out. I feel like there's a giant piece of hidden land I'm not seeing. Can't figure out how to get to the castle and haunted woods.

Follow the road east out of pluckmuckel for many screens to get to the haunted woods. The castle is some way beyond the woods. Youre right, there is a large section of the game that youve not found yet.

The turtles are a good way to grind out that last level's worth of XP after you complete the main game.  Just equip the spinning sword, fill that justice meter, and saw your way through a few dozen of them.  Outhouse away and back and repeat.  Won't take long to clear out that last cheevo.

Finished the game now, might try to get to level 20 tomorrow (im 18 now), don't really feel like it's worth it now though...

I finished every thing except the eubricks friends journal quest. 
 
I've went to the Demon Mines a bunch of times and still no luck...did my game glitch?