Why does my aggron keep getting destroyed by psychic types?

Question

everytime I fight a psychic type with my aggron. the psychic will take between a 1/4 to 1/2 my health per hit. Is there any way I can beef up my aggron?

Answer 1

Aggron has a low Special Defense stat. Only base 60. That is low.
To beef it up, train it more in SDef. How? Feed it Zinc, which raises SDef by a pinch.
Also, you can Surf the waters of Hoenn and kill Tentacool, which give off Special Defense EVs. If you kill 252 Tentacool, you will have a pretty high Special Defense stat (for an Aggron).