If it's just for fun you shouldn't be asking us haha. We can't tell you how to have fun.
I don't think it's a terrible idea to use that Granbull as a defender. However, I would use as is rather than powering it up. It's good on paper, but has such low bulk that most neutral counters will destroy it.
It's kind of like Gardevoir, in that Gardevoir is good for countering the meta but kind of frail against generalists, except Granbull both lacks the typing to truly "wall" off any and all counters (For example, I usually will champ through a Granbull with my Machamp if I'm sweeping), has even lower defense, and doesn't have a great defending Quick Move like Gardevoir has. Snarl is good, but it's what I would call a "medium-speed move" at 1.1s cooldown. The absolute best defenders use "slow-speed" moves, such as Confusion and Volt Switch.
