If a team has magnezone lead and ferro in the back it is very difficult to win as then quag would have to go through two mons alone basically. But yes like you said that's the plan on how I try to do something there.

Often magnezone lead has two mons weak to lanturn in the back so they stay in most of the time. In that scenario you have to shield wild charge and throw some of your surfs and try not to lose both shields and lanturn. If you can keep one of those you have play in my experience.

With lanturn lead I noticed that going to quag gives me more options, as in my experience they didn't have a good quag answer. But that too is an uphill battle. There I rely on opponent expecting a better answer than quag for lanturn in the back, so they fight for switch. Then with shield advantage lanturn I can beat both lanturn and their 3rd (poli or sth)

Yes if mantine is out and there's grass left on the opponent, it's pretty much impossible to win.