You are in luck, my friend! I've trained over two-dozen Shuckle over the past 10 years, and I may say that I am the Shuckle MASTER.
Swagger, swagger, swagger, that's all I can say. Shuckle has a monstrous Base 230 Defense Stat, so don't even be scared of maxing Swagger. The opponent will just beat itself senseless, if you're lucky.
Bide it up, Bro. When paired with a Swagger, the opponent will do a fair amount of damage to you. You'll take about 40 HP damage every Bide, and 80 HP damage isn't too bad for a Shuckle!
Rest. Shuckle has the immense survivability to take hits, even when he's sleeping, so Rest, in this case, is "reliable recovery."
Now you choose either Earthquake, Stone Edge, or Gyro Ball. All three are great attacks, but you can only choose one.
One tip for this cutesy turtle: DO NOT FALL VICTIM TO POWER TRICK. It sounds sweet, but what you really get is the most power physical sweeper in the game, with the lowest HP and Speed in the game. Do NOT, under ANY CIRCUMSTANCES, EVER use Power Trick. Sorry for the caps. I wanted to make sure you understood. :P
This is my advice, and is a collection of my 10 years of training these little guys. I have a Lv. 88 Shuckle sitting on my Gamecube in Pokemon XD, and I took out the Shadow Dragonite with it by accident, back when it was only Lv. 59. Yeah. Shuckle's that awesome. :D