Hey guys I get asked about tips for completing sidequests from time to time so thought I'd share some advice for completing them as quickly as possible. Here are a few tips, but the first is by far the most important.
Use Level 100 Dark (not the type) Pokemon with STAB moves >= 128 power
Moves like this will always do over 400 damage on a neutral target, meaning you can OHKO anything that doesn't resist the attack. Click the link below for a list of all moves that fall into this. Note: OHKO moves are excluded as they have 30% accuracy.
You can see that Ghost/Bug/Poison/Ground Pokémon are intrinsically worse due to not having STAB attacks that can OHKO.
The following Pokémon learn two different-typed moves from above as STAB (Best moves listed). I would recommend using these to make up the bulk of your team, as the extra coverage will help smash through the opponents. Value is placed on accuracy vs power. These are also roughly sorted by their type coverage (e.g. Reshiram & Turtonator hit more Normal/Super Effective and less No Effect than Chesnaught).
So... how do you choose your team using this? I use this coverage calculator for a rough guide on effectiveness. Using this, we can see that Fire/Dragon has brilliant neutral coverage, while Fighting/Flying has great super effective coverage. Therefore a fire/dragon and a fighting/flying type should be invaluable. As Shell Trap > Blue Flare, I would say that Hawlucha/Turtonator are essential. On the other side of things, Normal type attacks are less valuable as they hit nothing for SE damage, while being resisted by 2 and immune from another.
Type variance is key: you don't want to be walled by anything, while maximising super effectiveness (for shinies/metallics)
The following is just theorycrafting a team, may or may not be useful.
Use a dark-dark type with Fling as your first Pokemon, not much resists it and so you will be able to tear through most of the sidequests without even having to think.
As dark types are resisted by Fairy/Fighting/Dark; something that can deal with those after is generally useful and will enable you to keep clicking if your first faints. I personally use Greninja-Turtonator as my first two. But Shiftr-Turtonator should work just as well for a budget option.
Coverage on your Pokémon is great but not essential
See what each Pokémon/your team needs help with and work around that. Consider the level 120/150s within the sidequests: a lot will be Flying/Psychic/Dragon types so Ice/Dark coverage would go a long way to help beating those. However, if you have the >128BP moves, you can save money by not buying coverage moves as you won't need them particularly often.
Also, here are the magnetic field quest numbers for anyone needing them:
Hope this helps people doing sidequests. It's a horrible grind and the rewards aren't worth it unless you get beast balls/fossils, but it still needs doing unfortunately!
Feel free to message me for any clarifications or if I've made a mistake!