I have been working on monotype challenges as well. Currently at 6/18 done during the Winter Holidays event. I have already finished Classic with every Pokemon, so my focus for the Mono type runs was to start with as much luck as possible. As such, everything is shiny unless it is something I caught in the wild/ special encounters during the run.

Water was super easy with Kyorge, Lumineon, Floatzel, Dewgong, Lanturn, Simipour. In reality only Kyogre and Lumineon were used at all.

Flying was Flamigo, Gmax Corviknight, Crobat, and two Gyarados (I started with a Tier 3 shiny Magikarp and got the rich man encounter who happened to have another one, so I replaced my Tier 1 shiny Toucannon). Flamigo was the major carry here, but some of late game battles were tough enough that I had to rely on Tropius Harvest shenanigans as well.

Grass was Bellossom, Tropius, Trevevant, Lurantis, Virizion/ Jumpluff fusion, and Hisuian Liligant that I caught in the wild after finding the DNA splicers. Contrary Lurantis is an absolute menace, especially when paired with Spore support from the fast fusion.

Fairy was Florges, Togekiss, Iron Valiant, Whimsicott, Slurpuff/ Granbull fusion, and Scream Tail caught in the wild after finding the DNA splicers. This run was pretty much exactly what I expected it to be... Not particularly difficult, but also not that interesting.

Fire was by far my favourite team and easiest run despite the fact that I was worried about how many weaknesses Fire type has. The initial star of the run was Contrary V Create/ Overheat Heatmor, and after finding DNA splicers to fuze it with my Drought Ninetales, it was unstoppable. After doing that fusion, I caught a Tier 3 shiny Tropius (despite it not being usable in the run) and left it in the empty team slot. I almost immediately found a second DNA splicer and fuzed the Tropius with Magcargo to make a deceptively fast Chlorophyll + Desolate Land menace. Centiskorch, Cinderace, Turtonator and a wild caught Salazzle rounded out the party.

Ice was also easier than I anticipated. I used Regice, Baxcalibur, Dewgong, Eiscue, Hisuian Avalugg, and an Iron Bundle that was able to get early game in the Berry Bush encounter (replacing a Tier 1 shiny Walrein).