Speaking of not-quite-imposter resistant, I present to you;
Degrees of Imposter-proofing for a single Pokemon
1: Completely immune
A Pokemon fits this tier if, and only if, an Imposter can literally do nothing to it. If it can take on six Imposters in a row and win without trying, it fits this. Examples; Ghost-types with Spooky Plate Judgment, the Kyurem directly above this post on at least one boost. A Pokemon may still loose to an Imposter if afflicted with an outside status from another Pokemon, such as Leech Seed or Burn, or if the Imposter is holding an appropriate item.
2: Very resistant
A Pokemon fits this tier if it can be pitted against a single Imposter while in relatively poor condition and be able to either force a switch or a KO. Examples of this include Normal-resisting Bulletproof Pokemon with Judgment and a selection of moves affected by such, Pokemon with enough bulk to resist their own attacks to the point that the Imposter runs out of PP (EG: Unaware Darm-Z w/Fire move and Psystrike), bulky Poison Healers, Safety Goggles wearing Spore users, the Contrary Giratina directly above.. If something is weakened to a point at which it would be practically fodder anyway (Darmanitan-z weakened and Toxic-poisoned) it may still loose, or if it has to face multiple Imposters.
3: Resistant
To fit this level, a Pokemon in good or perfect condition can face an Imposter in a one-on-one match and defeat it. Examples include Poison Heal Regigigas, Magic Bounce pokemon with only moves reflected by MB, Pokemon that have already used Imprison, critical-spam users with offensive stats already dropped, Judgment users that have an SE Judgment but don't resist Normal, Belly Drum+Unburden combos, all Illusionists with the Illusion intact. If it is already weakened or the Imposter user lucks out with hax then it is possible to loose a matchup with this level. Magic Bouncers with the right moveset belong here as they can both do nothing and have nothing done to them. If a Pokemon is completely immune to it's entire moveset but cannot hurt the Imposter in return it also belongs here as it can force an Imposter out once or twice due to lack of PP but cannot actually do anything about it.
4: 50/50
If a Pokemon has a chance to win against a (Non-Scarfed) Imposter if it wins a Speed tie it belongs here. Examples include set-up users with an appropriately powerful move (Belly Drum and a move the user is weak to, Shell Smash Aerodactyl-Mega with Head Smash etc) Special Contrary users boosted to a high enough level, some Protean sets and Imprison users that haven't set it up already.
5: Weak
Something on this level will likely be defeated by an Imposter. This includes set-up without a strong enough move to OHKO, most Protean sets, Physical Contrary, unboosted offense users and most support Pokemon not carrying Knock Off. If you get lucky you might be able to defeat the Imposter, or if it's already weakened to a sufficient degree, but you'll most likely loose out.
6. Why did you even try?
Something that goes here can never beat an Imposter. Ever. Full stop. Pokemon that are always OHKOed by their own attacks and cannot ever OHKO in return, a Pokemon that looses in a slugfest even if it only gets crits, etc. Don't even try with these, unless you're weakening the Imposter or deliberately sacrificing the Pokemon currently out.
Other things; Remember, just because something looses one-on-one it doesn't mean it's a bad choice. Just remind yourself to keep a hard-counter on your team and you'll do fine. Also take into account the fact that Imposters are capable of holding items other than Eviolite and Leftovers, so your "Immune" Mega Gengar at +6 in every offensive stat may well be one-shotted by a Chansey carrying an Earth Plate.
BH is a ridiculously wide tier. I haven't thought of everything. Any suggestions will be added to the appropriate area (Or as a new area), so please make them.