First things first I don't know crap about competitive Pokémon, but I'm willing to learn. In this episode I try to learn how to use Mega Venusaur. I mostly just use Smogon sets since I'm a beginner. Don't consider this a Mega Venusaur guide, because I'm not at that level. I'm trash lol.
Since Mega Venusaur is OU I battled in that tier. I used this set for my Mega Venusaur, aptly named Groot; Giga Drain, Sludge Bomb Synthesis and Knock Off.
My Mega Venusaur was Calm nature using these spreads 248 HP/88 Def/156 SpD/16 Spe.
Mega Venusaur's very good bulk allows it to check a plethora of prominent threats in the OU metagame, including Tapu Koko, Keldeo, Battle Bond Greninja pre-transformation, Protean Greninja without Extrasensory, and Tapu Bulu.
Its Thick Fat ability and decent defensive typing leave it with only two weaknesses, Psychic and Flying, while giving it resistances to many common types, such as Water and Fairy. Mega Venusaur is surprisingly not very passive, as it possesses a solid Special Attack stat of base 122.
Its ability to run either an offensive or defensive set lets it fit on a variety of team archetypes. However, Mega Venusaur faces massive competition from Tangrowth and Amoonguss, both of which perform similar roles and don't need to run Synthesis because they have Regenerator.
Also, it takes up a Mega slot, preventing the player from using generally better Mega Evolutions such as Mega Mawile, Mega Medicham, and Mega Scizor. Mega Venusaur is easily worn down by Spikes, Scald burns, and even just powerful attacks, so it finds itself using Synthesis a lot as opposed to attacking, which results in momentum loss and passivity.
Finally, it gives a free switch in to certain common Pokemon, such as Autotomize Celesteela, Gengar, and Mega Medicham to an extent, which can spam their powerful STAB moves or set up on Mega Venusaur.