Deoxys-Attack arrives in regular Tier 5 Raids alongside its other 3 forms in the second week of the 2019 Ultra Unlock. Deoxys-A is an extreme glass cannon that makes Gengar look tanky. It has 414 Base Attack, which dwarfs any other Pokemon in the game. However, it's so glassy that it will faint in 4 Confusions from a Mewtwo Raid Boss; 414 Base Attack won't mean a whole lot if Deoxys can't even get one Charge Move off.
Thanks to Deoxys-A having lower Base Defense than Caterpie, it is by far the easiest Tier 5 Raid Boss. It is so fragile that it can be soloed.
If you have never soloed a Raid Boss before, you may not be aware that Raid Bosses regenerate HP if there is no one actively fighting it.
What this means: If your 6 Pokemon all faint and you are forced to revive, the Raid Boss will regain some HP when you rejoin, which can make soloing Raids significantly harder.
Minimum viable levels assume No Friendship, No Weather, and under 12 total Faints with No Dodging.
Note: All calculations assume regular Tier 5 HP, and not the boosted "Tier 6" HP that Mewtwo received in 2018.
Italics indicate a Legacy move.
The Pokemon listed here are capable of soloing Deoxys-A without dodging. If you are capable of consistently dodging, Gengar becomes a top-tier counter capable of easily soloing Deoxys, even against Psycho Boost.
Without dodging, only Giratina-O and Tyranitar are capable of soloing Zap Cannon Deoxys.
If you are able to consistently dodge, Zap Cannon becomes drastically easier to solo. Traditional Dark- and Ghost-type counters such as Gengar, Weavile, and Shadow Ball Mewtwo can easily solo Zap Cannon Deoxys with dodging around the Level 30-35 range.
Only Tyranitar and Weavile are capable of consistently soloing Dark Pulse Deoxys without dodging.
Again, however, if you are able to consistently dodge, many more options become available - Gengar, Giratina-O, Shadow Ball Mewtwo, Houndoom, Honchkrow, etc. are all perfectly viable options if you can dodge.
If you don't know how to dodge and would like to learn how, check out our Dodging guide.