Ok so ummm I'm probably late but I'm going to answer this as best as I can. I asked on Smogon a while back, and this is the answer I got from a National Dex Monotype player named AA (formerly Attitude Adjuster),
Most types have counters or ways to offensively counter Pokemon, for example having an Electric immunity is key like mega Swampert who is sadly slower than Raichu-Alola, but you can also run Barraskewda who 2hkos and outspeeds almost everything on electric but is sadly harder to fit on most teams, this replay is a great example of water beating electric: Replay of Water Mono beating an Electric Mono
In the replay, Decem, who had the mono water team, won against a mono electric team. Anticipating the moves the opposing Pokémon had was key to victory. Like what the previous comments said, having counters to the type you're at a disadvantage (i.e using Mega Swampert on Mono Water) also helps. Even if it's something like offensively countering Pokémon of the opposing like what the Smogon user AA said, it also can help you beat a type disadvantage. Pokemon with Dual types, like Assualt said, also helps with dealing with type disadvantages, as dual types Pokemon can provide a key immunity or immunity core (i.e the steel immunity core which uses Heatran, Aegislash, and a steel/flying type) for a type. Moves that can provide coverage and sometimes Abilities (like Flash Fire on Heatran) can also play a role in helping you beat a type disadvantage.
To sum this up, anticipating what moves opposing Pokémon have, having Pokemon that can provide immunities and/or resists, being able to counter Pokemon (even if it's offensively countering), moves that provide coverage and Abilities all play a role in being able to beat a type disadvantage. If I missed anything, let me know.
Hope this helps.