With the ability to take both physical and special hits somewhat well combined with the ability to support its team via healing status effects and phazing, Meganium has what it takes to be a good supporter for its team. Aromatherapy lets Meganium cure its team of status, possibly giving teammates a second chance if they were crippled by it. It also ensures Meganium is not worn down by Toxic, which would ruin its staying power otherwise. Dragon Tail allows Meganium to phaze opponents, and it works well with entry hazards in order to rack up damage. Phazing also prevents Meganium from being setup fodder for sweepers unless they are using Substitute. Giga Drain is Meganium's STAB move of choice and lets Meganium keep itself healthy while damaging foes such as Kabutops. For the final slot, Synthesis provides useful instant recovery to let Meganium last longer, but Toxic is a viable alternative in order to wear down other Pokemon.
A specially defensive spread is chosen in order to allow Meganium to take moves such as Scald and Volt Switch better, as well as an Ice coverage move if the need arises. Although Meganium's Dragon Tail suffers from a Calm nature, avoiding this is not worth lowering Meganium's surprisingly decent Speed stat that can allow it to use Aromatherapy in a pinch, and the amount of damage dealt with Dragon Tail is not overly important as it is not meant to hit very hard in the first place and struggles to break Substitutes regardless. A physically defensive nature is usable to sponge Earthquakes and Waterfalls more easily, but as most of Meganium's resistances are primarily used as special attacks, a corresponding EV spread comes in handy more often. There are other moves worth considering that can occasionally come in handy. Leech Seed is always a cool option in order to wear down the foe while getting recovery, but unfortunately, Dragon Tail cancels this effect. Meganium can also support its team via dual screens, but as that means giving up Aromatherapy, Dragon Tail, or Giga Drain, it can be tough to fit both moves onto Meganium. Finally, Grass Whistle has the reward of inflicting the always-useful sleep, but its poor accuracy makes it risky, and there are many sleep absorbers that already get free switches into Meganium.
As Dragon Tail itself isn't the most powerful attack, entry hazard support is useful as the passive damage makes phazing much more devastating. Toxic Spikes in particular can be very useful as it allows Meganium to run Synthesis over Toxic without any problems. As a result, Qwilfish makes a very good partner, as it is an effective dual Spiker and can take Bug and Fire moves that might be directed at Meganium. Defensive Omastar can play a similar role, except it doesn't resist Bug. It does, however, get access to Stealth Rock to wear down Fire-type switch-ins. Alomomola and Poliwrath can also switch into Fire attacks and provide support for Meganium; Alomomola can provide Wish support, which helps Meganium if it's running Toxic and, as a result, has no instant recovery. Poliwrath can switch into Durant and Escavalier without much of a problem, and can also provide additional phazing via Circle Throw. Poliwrath also appreciates Aromatherapy support to make its typical RestTalk strategy more reliable. Finally, Escavalier and Durant cover many of Meganium's weaknesses and can usually destroy some of Meganium's common checks, such as Amoonguss, Cryogonal, and Roselia. Both also become utterly useless if inflicted with a burn (and for Durant, paralysis as well), so they appreciate Aromatherapy giving them a second chance. Note that the two weaknesses Bug and Steel do not cover, Flying and Fire, can be worn down by Stealth Rock, and Flying STAB moves are uncommon in RU.