You can raise your Pichu's Defense by giving it Iron, using Poke Pelago's Isle Evelup, or by having it in the party with Exp. Share on while you beat up Geodudes and other Pokemon that give Defense EVs.
That said, Pichu and its evolutions don't have good defense to begin with, so you're probably better off focusing on its strengths.
Pichu and the like all have bad physical defense. I found that out the hard way. Other than try to train EVs, I would try to catch one with a nature that bolsters its attack and speed for best results.