I'm looking to cut through a standing pantry for an electrical box. The pantry panel is made of 3/4" plywood. I attached a proper HCS blade to an oscillating multitool (Ryobi) to start with but it's very difficult to cut through. I was able to finish the job with drilling multiple holes and hammer it through. The result is very rough with this method and I'm hoping to find a better solution. Is there a better mobile tool that could help in such cases?
A cutout tool like the Dewalt DW660 works amazingly well for cutting out holes for things like electrical boxes. I can't remember if it comes with bits for wood, or only for drywall, but wood bits are readily available, like these ones: RotoZip SC16.
You just plunge the bit into the material and move it around the outline you want cut. Very similar to using a jigsaw, except it can make hairpin turns.
As noted in the comments, I'd suggest a jig saw.
You can get a fine tooth blade that should give you a reasonably nice finish on the cut (though it will cut slower) and it will be easier to control than a reciprocating saw (Sawzall™ type saw). In my experience, plunge cutting with a reciprocating saw usually gives me a bent blade (I'm sure if I had more experience, I'd do better).
Using a circular saw will work, but plunge cutting can be difficult if you're not used to it especially if you're cutting a vertical surface and cramped inside a cabinet box with little room to work and limited lighting, and by the time you've got the blade set shallow enough to not go too deep past the other side of the plywood box, you're really not going to be cutting too much of the wood (due to the arc of the blade vs the flat wood) and will probably need to finish with a jig saw anyway.