Removing out DLC 2:
/challenge XScipio, gen8ou@@@ -kubfu, -urshifu, -calyrex, -regieleki, -regidrago, -glastrier, -articuno-galar, -moltres-galar, -Zapdos-galar, -slowking-galar, -nidoking, -nidoqueen, -crobat, -jynx, -electivire, -magmortar, -omastar, -kabutops, -aerodactyl, -Zapdos, -moltres, -articuno, -dragonite, -raikou, -suicune, -entei, -sceptile, -blaziken, -swampert, -aggron, -altaria, -cradily, -armaldo, -absol, -walrein, -relicanth, -salamence, -metagross, -regirock, -regice, -registeel, -latias, -latios, -spiritomb, -garchomp, -uxie, -mesprit, -Azelf, -heatran, -regigigas, -cresselia, -victini, -audino, -carracosta, -archeops, -cryogonal, -tornadus-therian, -thundurus-therian, -landorus-therian, -genesect, -tyrantrum, -aurorus, -carbink, -diancie, -volcanion, -tapu koko, -tapu lele, -tapu bulu, -tapu fini, -nihilego, -buzzwole, -xurkitree, -celesteela, -kartana, -guzzlord, -poipole, -naganadel, -stakataka, -blacephalon, -expanding force, -steel roller, -scale shot, -meteor beam, -misty explosion, -grassy glide, -rising voltage, -terrain pulse, -skitter smack, -burning jealousy, -lash out, -poltergeist, -corrosive gas, -coaching, -flip turn, -triple axel, -dual wingbeat, -scorching sands
Removing out DLC 1:
/challenge XScipio, gen8ou@@@ -kubfu, -urshifu, -calyrex, -regieleki, -regidrago, -glastrier, -articuno-galar, -moltres-galar, -Zapdos-galar, -slowking-galar, -nidoking, -nidoqueen, -crobat, -jynx, -electivire, -magmortar, -omastar, -kabutops, -aerodactyl, -Zapdos, -moltres, -articuno, -dragonite, -raikou, -suicune, -entei, -sceptile, -blaziken, -swampert, -aggron, -altaria, -cradily, -armaldo, -absol, -walrein, -relicanth, -salamence, -metagross, -regirock, -regice, -registeel, -latias, -latios, -spiritomb, -garchomp, -uxie, -mesprit, -Azelf, -heatran, -regigigas, -cresselia, -victini, -audino, -carracosta, -archeops, -cryogonal, -tornadus-therian, -thundurus-therian, -landorus-therian, -genesect, -tyrantrum, -aurorus, -carbink, -diancie, -volcanion, -tapu koko, -tapu lele, -tapu bulu, -tapu fini, -nihilego, -buzzwole, -xurkitree, -celesteela, -kartana, -guzzlord, -poipole, -naganadel, -stakataka, -blacephalon, -expanding force, -steel roller, -scale shot, -meteor beam, -misty explosion, -grassy glide, -rising voltage, -terrain pulse, -skitter smack, -burning jealousy, -lash out, -poltergeist, -corrosive gas, -coaching, -flip turn, -triple axel, -dual wingbeat, -scorching sands, -Slowpoke-Galar, -Slowbro-Galar, -Zarude, -slowpoke, -slowbro, -slowking, -lopunny, -chansey, -blissey, -wigglytuff, -lurantis, -talonflame, -luxray, -klefki, -alakazam, -tentacruel, -dunsparce, -bouffalant, -lickilicky, -druddigon, -scolipede, -amoonguss, -comfey, -tangrowth, -Zoroark, -starmie, -emolga, -dedenne, -Magnezone, -sharpedo, -stoutland, -miltank, -scyther, -Scizor, -pinsir, -heracross, -palossand, -Azumarill, -poliwag, -poliwrath, -politoed, -golduck, -exploud, -skarmory, -lycanroc, -mienshao, -sandshrew, -sandshrew-alola, -sandslash, -sandslash-alola, -marowak, -marowak-alola, -kangaskhan, -krookodile, -volcarona, -dragalge, -clawitzer, -seadra, -kingdra, -lilligant, -exeggutor, -exeggutor-alola, -porygon2, -Porygon-Z, -magearna, -libero, -sniper
Phew, it was long and tedious. Replace "XScipio" with the desired username and "gen8ou" with the desired format. Note you can only do this by challenging somebody, you can't do on ladder, though.
Hope this helps!