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Monograss is one of the worst monotypes, they have so little diversity in movepool and secondary typings.
If you aren't playing by smogon fag rules, you basically need to devote half or more of your team solely to defeating talonflame, because a good talonflame user is going to hit and run, even if you have one or two team members in line for it.
Tangrowth can tank a hit from talonflame and KO in return, Cradily is also mandatory for flying and bug types which grass types eat ass against. But seriously, talonflame is your main priority, I ran monobug for a while and even a mediocre talonflame user would take out half of my team before I could bring it down. You can't just say "oh I have X pokemon who can take a flare blitz, so I don't fear talonflame!" You need a pokemon that can reliably tank two flare blitzes, and ideally two rounds of stealth rock as well, because they're not going to just leave a valuable talonflame in on a full health tangrowth/cradily to let it die. they're going to u turn out, and every time you want to switch tangrowth/cradily in they're going to have to take another flare blitz. Stealth rocks is also mandatory to reduce this as much as possible, so hopefully you'll only have to take two attacks from talonflame after recoil if they're a uturn fag
Mega venusaur vs mega sceptile could go for what your team needs more. I kind of want to recommend veusaur myself, but mega sceptile can help if you have trouble with dragons.
Celebi is probably mandatory, I don't have a lot of experience with it, but it has a useful movepool which the type is lacking
I'd almost advise against serperior. As good as powerful leaf storm spam may sound, you'll probably have more than enough grass type attacks, you don't want to be saddled with serperior's poor defenses
Gourgeist has an interesting movepool and typing, will o wisp is very useful, I'd strongly consider