Shuckle is an odd Pokémon, but it can still be a pain if not treated seriously. It has the highest defensive stats in the game, making it almost impervious to measly little attacks - even super effective ones. The first Gym Leader, Annette, has one as her ace and it's nothing to be scoffed at.
With this being said, Shuckle's HP and attacking stats are extremely pitiful so it cannot dish out the damage nor tank hard hits as well as one thinks. It is also the slowest Pokémon in the game with a Speed stat of only 5.
In the beginning this thing can be an absolute pain to beat, especially as it's Level 15 in the Gym Leader battle, but once more Pokémon become available there's really no need to use it. Unless one wants the highest defensive stats on their team as a stalling machine with Leech Seed and Spikes, of course.