If you have a Mold Breaker Basculin that knows Soak and Hidden Power for Electric, those two moves by themselves are capable of combining for super-effective coverage against almost everything in the game, foiled only by Arceus and Silvally (which are immune to type changes by any means, even from a Mold Breaker user), as well as Primal Groudon (which blocks Soak by means of a weather condition, which exists as a separate entity to the ability that created that weather). Granted, it's not a very strong super-effective hit, and it takes two turns to pull off in most cases, but that just goes to show that "full type coverage" isn't all it's cracked up to be.
Of course, Basculin does still have two open moveslots after that, and the best it can do with those to maximize coverage against the remaining holdouts is Superpower + Mud Shot. This takes care of the Primal Groudon case, as well as 10 different types of Arceus and Silvally (including the normal-type version to cover every item except their specialty items).
Since the only holdouts through that are 8 different types of Arceus and Silvally, all of which conveniently only require considering monotype opponents instead of dual-type combos, it's simple enough to concoct a moveset that can cover all 8 super-effectively using just one additional Pokemon. An Octillery with Ice Beam/Thief/Bounce/Sludge Bomb, for example, would fit the bill.