If you are choosing to grind for the E4, then you have some options:
For the Bottom or Outside of Mt. Lanakila, I'd recommend giving Silvally the Fighting-type memory.
For the Inside of Mt. Lanakila, I'd recommend the same as above (if you aren't facing a Golbat), but if you are, make sure your Silvally has a Rock-type move as one of its 4 moves (I wouldn't recommend the Rock-type memory due to the abundance of Normal- and Ice-types).
Note that there are plenty of Normal-, Ice-, and Dark-types there to grind on (the exception being Golbat). So, use the Fighting-type memory and have a Rock-type move in your arsenal. Good luck.