Well things that are set in stone, such as nature are a big part of how slow you are. You'd want a nature that makes you slower, best one for groudon being brave or relaxed. Also minimum IVS, which is really hard to do with the nature and all that and I'd recommend ignoring this part tbh. Something that you can easily control are EVs. Don't invest in any speed EV's. If you have already, eat plenty of tamato berries until it goes down to zero. That should help make groudon slower
