Hiya!
So, the other day I was in the battle factory in platinum, and a meganium comes out with a moveset that I found incredible. I decided to get a pokemon on my battling team replaced with another with a similar moveset. Now, I have one worry: it is that this moveset has some huge hole that I have yet to see. Well, here it is:

Substitue
Toxic
Giga Drain
Leech Seed

Now, the NPC didn't execute the strategy as I believe it was mean to be. I think that the moves should be used like so:
Substitute, Toxic, Leech Seed, Giga Drain until dead. Then Substitute when the doll dies. (wait what? you know what I mean.......i hope...) Are there any holes in this strategy? Because the way I see it, Megainium sacrifices health, then gains it all back with GigaD/LeechS, right? And Toxic is annoying, so feel free to poke holes in this strategy. thanks

Steel-types. Really all that can be done to them is leech seed, while they can blast through your subs. Grass/poison types will wall almost entirely as well as Tentacruel and Swalot w/ Liquid ooze. Taunt shuts down all but giga drain, meganium is towards the slow side so many users of taunt will outspeed. Sub users fit in the same boat as well, as it blocks leech seed and toxic as well as giga drain's low power may not break a subs in one hit.

Also any fast pokemon with high attack that knows either a fire, ice or flying move, which are all very common in the wifi battling.

On the contrary, SubSeed is quite effective, because your opponent has to take a turn to break the sub, exposing it to Leech Seed. The basic idea is that you switch in, Substitute while your opponent switches out or using a slower status move, then Leech Seed. If your opponent stays in to kill the Substitute, they get Seeded, and you can keep Substituting by using the HP regeneration from Leech Seed. If your opponent switches, you have a free turn to remake your Substitute or Seed the switch-in.

If the rest of your team is more defensively oriented, Toxic can be a nice bonus.

As previously noted, though, fast pokemon will shut down Meganium with Taunt, or just outspeeding it and attacking before it can remake its Substitute. SubSeed is more commonly run on Sceptile.