Darkrai appears in Raids for Halloween 2019, and it is an amazing Pokemon. Base 285 Attack and access to Snarl/Shadow Ball lands Darkrai at the top of Ghost/Dark-type performance. While it's squishier than Giratina-O, its Dark-typing means it resists Ghost and is immune to Psychic, making it actually bulkier than Giratina in many matchups.
With Niantic's Halloween surprise of a Tier 6 Darkrai, it is much more difficult to take down than its Tier 5 variant. The trio is possible, but requires Cloudy weather; the 4-man can be quite difficult with low-level counters, but 4-manning with high-level Machamps is quite easy.
Minimum viable levels assume Best Friends, Cloudy Weather, and no Dodging.
Italics indicate a Legacy move.
Level 40 Counter/Focus Blast Blaziken, Counter/Close Combat Heracross, Counter/Dynamic Punch Toxicroak, and Charm/Dazzling Gleam Togekiss are slightly short of duo-viability, but, with enough Machamps, can be part of a successful trio composition.
Level 40 Charm/Dazzling Gleam Togekiss and Counter/Dynamic Punch Toxicroak are slightly too weak to be duo-viable against Focus Blast. Again, if you have enough Machamps, you can afford to slot in a few of these Pokemon without throwing your chances of success.
Shadow Ball hits hard enough to force 2 relobbies. If you're using mostly Machamp, dodging Shadow Balls will allow you to avoid relobbying entirely; with Breloom, dodging will allow you to relobby only once, making the trio significantly more comfortable.
Minimum viable levels assume Best Friends, no Weather, and no Dodging.
Italics indicate a Legacy move.
Dark Pulse is the easiest moveset, and should be a fairly straightforward fight. Dodging will lowers Machamp's DPS more than the extra survability is worth, but will save your group many Revives in the long run.
Fairies do well against Focus Blast, thanks to their resistance to Fighting. Gardevoir and Togekiss in particular enjoy a 4x resistance to Focus Blast, making them very Revive-efficient options.
Breloom is non-viable against Focus Blast due to its squishiness; however, if you can dodge, Level 30 Breloom is strong enough to 4-man Focus Blast Darkrai.
Level 40 Counter/Close Combat Heracross and Low Kick/Focus Blast Emboar are slightly too weak to be viable against Shadow Ball, but with enough Machamp can be slotted into a winning composition.
Shadow Ball hits hard enough to force 3 relobbies without dodging. If you can consistently dodge Shadow Balls, you will be able to cut that down to 1 relobby, which will make the fight significantly more comfortable.