Mixing it up is crucial, yeah.
Can anyone tell me what to do against ROB? ...I'm having problems with him.
ROB is evenish, kinda our advantage imo. I used to have trouble with the mu too.
First off I recommend not camping too much. Nades are NOT the best in this MU though there are some uses for them (cause his laser spam blows up nades in your face). Camping is kinda evenish, though I think ROB has the advantage. You may want to slowly approach. Your best thing vs ROB is to grab and tech chase - his rolls are kinda short, and his getup attack isn't too hard to punish either. Watch out for his ftilt and and fair... he has good range with those, and beware of the gyro and laser. Those shouldn't hit you too often if you're watching him.
ROB is really good at punishing your recovery. Be careful, and try to recover high if you can as much as possible. Also, you can try double jump nairs and full hop bairs now and then to block his aerial approaches. ROB cannot airdodge while doing up b either. Take advantage of that now and then.
ROB usually likes to kill with nair, bair and fsmash. Occasionally, usmash (though it's tricky to land), fully charged laser when you're recovering, and fully charged gyro near the sides of the stage will kill you as well. Usmash is his most powerful killing move too, so watch your landing.
ROBs like to use spotdodge -> dsmash, and some of them will getup from your dthrow and do a dsmash right away. Shield it all, and then grab. It has a lot of hits. Also beware ROB's grab. If he grabs you, hold up, cause his fthrow and bthrow are really fast and if you DI wrong it can put you in a bad spot, or kill you. The other throws are kinda slow so you can just react if he does one of them. I think ROB's shield is kinda small too, so abuse.
He dies a bit over 100% reliably from c4/uair/utilt. I've heard some people really recommend a lot of c4 in this MU too.
TL;DR Vs ROB, be ready to approach, tech chase him a lot, and don't use a lot of nades.
If other Snakes want to add input, go ahead. I think most of this stuff is correct but I'm sure there are players who know more about this MU than I do.