First off, Memento + Magby, Screens + Magby, Larvesta + Magby or any of the wide varieties of strategies people use to deal with Magby are impossible to deal with for everyone, including the top players. Magby is just an absolute nightmare mon in general. There are some counters though
Both of these pokemon are immune to Mach Punch, and with a Choice Scarf outspeed and kill Magby. I think Scarf Frillish is a little bad but Scarf Golett is one of my favourite choices for Magby weak teams.
Onix lives a Mach Punch from full even without Eviolite and kills back.
You mentioned this one already, Evio Diglett outspeeds and lives mach punch
All these mons hit 19 speed, matching Magby and can OHKO it. Magby of course can also one-shot them, so its a 50/50 and unreliable. if your Abra is sash then you beat it no matter what though, which is nice.
Getting hazards up obviously puts Magby under a lot of pressure due to it's frailty and fire typing.
Belly Drum taking a lot of your HP means that Magby can often be taken down by Fake Outs, Mach Punches and Grassy Glide.
***
There are some other counters, such as using Focus Sash pokemon, but overall Magby is just really hard to deal with in general if it can set up though. You just have to try and position intelligently and not leave yourself in a position where Memento into Magby is completely free. Good luck.