252 HP / 240+ Def Mega Venusaur: 265-312 (72.8 - 85.7%) -- guaranteed 2HKO So, you're right. Bish doesn't OHKO with Sucker Punch <3 (p.s.) HUpa can also not run Hasty and run Timid instead, and have a full Special moveset. So, there goes ANY chance of an OHKO with ...
If Magnezone is choice-locked rather than carrying a resist berry, Dugtrio can potentially even use it for Stealth Rock/Substitute fodder depending on the moveset choice. If the team appreciates it, Choice Band Dugtrio and/or physically defensive Skarmory can also be used. Be careful of Knock ...