Reserving Lando-T and Mega Venusaur
Check:
Landorus-Therian (M) @ Groundium Z
Ability: Intimidate
EVs: 252 HP / 132 SpD / 124 Spe
Careful Nature
- Earthquake
- Stealth Rock
- U-turn
- Knock Off
Now, normally lando-t does not in fact check magearna. However, this lando-t set does. With these specific evs, you always live a twinkle tackle, meaning you can actually switch into any move that magearna has at neutral. From here, you can respond with a tectonic rage which cleanly ohko's magearna, meaning you don't have to deal with any of its shenanigans with pain split. Rest of the moveset is more supportive, and the speed evs are just the leftovers, though they of course can go into attack if wanted.
Counter:
Venusaur-Mega @ Venusaurite
Ability: Thick Fat
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Growth
- Earthquake
- Synthesis
- Leech Seed
Mega Venusaur is bulky AF. Just look at this calc:
+3 224+ SpA Magearna Twinkle Tackle (195 BP) vs. 252 HP / 252+ SpD Venusaur-Mega: 271-320 (74.4 - 87.9%) -- guaranteed 2HKO
Yes, it can live a +3 twinkle tackle from magearna, and synthesis off the damage even if it hits it on the switch as venusaur is faster than magearna. From here, magearna can't do much. Even at +6, fluer cannon is never ohko'ing venusaur and is getting weaker. Venusaur can earthquake the magearna for a 3hit ko, which magearna can try to outheal with pain split, but this doesn't do too much if the venusaur player plays it right. Leech seed can help venusaur to stay healthy passively, while growth helps it to boost up and ko the magearna, as at +1 e-quake 2hit ko's magearna.