I had a quick look to see how many attacks there were of each type. Bear in mind that this includes attacks that are not allowed to be called by Metronome, or will always fail (e.g. Sucker Punch).
18% of attacks are Normal type, which is why Ghost and Steel types are so popular. Ghost is even more popular because 8% of attacks are fighting type, so they're immune to over one quarter of attacks, while only being weak to 9% of attacks. On the other hand Steel resists 58% and is immune to Poison (3%), although it is weak to 20% of attacks, so you would want to consider a secondary type with a Fighting or maybe Fire resistance. At the other end of the scale, only 2% of attacks are Fairy type, which only helps Pokestar Spirit to be viable, as Fairy is its only weakness and it's also immune to Psychic, which takes it up to almost one third immunity. This assumes you're not up against Scrappy or Pixilate of course.
If you decide to use Abilities to give yourself more immunities, then the best single choice is Flash Fire, as 7% of attacks are Fire type, but of course Storm Drain (6%) and Lightning Rod (5%) also protect your ally from single-target moves, except Water Pledge. Of the other Abilities, Sap Sipper covers 6% and Levitate a mere 4% of attacks, so you would probably only want to use them if you have a double weakness.
Of the Abilities that reduce damage, Delta Stream halves damage from 13% of attacks, and protects both allies if they are both Flying type, but also protects your Flying type opponents, while Thick Fat halves damage from 11% of attacks and still allows a Weakness Policy to activate if appropriate.
Speaking of which, you might wonder how easy it is to trigger a Weakness Policy. In this case the best selection would seem to be Terrakion, which is weak to 36% of attacks, although usefully is actually resistant to 42% of attacks. (Perhaps paired with Prism Armor?) Another interesting possibility is Blacephalon, as its Fire typing's 15% weaknesses and 28% resists have no defensive synergy with its Ghost typing.
On the other hand, you could choose your typing to get as much STAB as possible for that juicy 50% extra damage. Scrappy Lopunny-Mega would seem to be a good fit, getting STAB 26% of the time. Perhaps you could run Aerilate Moltres, getting 60% combined STAB + Aerilate on 18% of hits and 50% STAB on 7% of hits. I know it's doubly weak to Rock but you don't have to worry about Stealth Rock and only 3% of attacks are Rock type.