A long-range MS (ex. Launcher Strike, Blast Impulse, Calamity, Buster, Destroy) can take down anything at extreme long range.
Beam sabers and combat knives can work frighteningly well against Grass-types like Venusaur; so can heat rods (GOUF Ignited) and spherical breakers (Raider). Venusaur's Solar Beam would be akin to a low-energy beam weapon blast; thus, it can be blocked with the proper safeguards (Phase Sihft, beam shields, et cetera).
Water wouldn't have any adverse effect on a mobile suit; in fact, there are quite a few mobile suits (ex. Abyss, Forbidden Blue, Deep Forbidden) that are designed for underwater operations. Even if it wasn't designed for aquatic combat, any MS can take on a Blastoise and win. Besides, the Forbidden's Geschmeidig Panzer energy reflectors can block any Pok?mon's attacks, thus rendering any attacks a moot point; any unit with beam shields or positron deflectors can also block attacks entirely.
Fire cannot harm Phase Shift, Trans Phase, laminated armor, or any other form of protection on an MS; all it'll cause is cosmetic damage. A unit with anti-beam coating or mirror coating (Akatsuki) can disperse fire.