I like betamon because no matter what you choose to go to it will always be a decently strong decently tanky magic guy with some variation between those two facets
in the second game they got smart and anything that's intended to be remotely tanky has a Pen Block in its items or is just outright given resistance to piercing effects
currently you have a pretty okay-ish matchup against data types, and can do the funny thing against viruses where you field only vaccines to completely smother them
generally it goes by jrpg rules, the latest and most difficult area you can access is generally the best place to grind in, though it definitely helps to have a guy for exp boosting