I'm not sure I agree with that.
Whilst Ferrothorn I do think you can deal with with the combination of heatran and azumarill, defensive mega venusaur is a real issue to the team. You are pretty much relying on the flinches to win. With your spread, you have a chance to 3hko with iron head. Actually you see this quite a lot in competitive games where people refuse to run zen headbutt because it's a "bad move" which it kind of is against a wide variety of the metagame, but it does leave these teams to getting 6-0ed by Venusaur. Venusaur I believe was 8-2 in BSPL USUM BSS because players did not respect or prepare for it within their teambuilding. Venusaur in my opinion beats every single mon in your team and can probably just lead turn 1 and win and it forces you to bring p2, metagross and probably greninja just because that one mon exists on the opponents side.
I can't say there's an easy answer to your issue and it's probably fine having that one bad matchup since venusaur isn't too common (as said above, you pretty much have to rely on hax to win), however replacing iron head is not that answer. Iron head however is pretty much mandatory and is easily metagross's best move, don't change it for anything.
Whilst Ferrothorn I do think you can deal with with the combination of heatran and azumarill, defensive mega venusaur is a real issue to the team. You are pretty much relying on the flinches to win. With your spread, you have a chance to 3hko with iron head. Actually you see this quite a lot in competitive games where people refuse to run zen headbutt because it's a "bad move" which it kind of is against a wide variety of the metagame, but it does leave these teams to getting 6-0ed by Venusaur. Venusaur I believe was 8-2 in BSPL USUM BSS because players did not respect or prepare for it within their teambuilding. Venusaur in my opinion beats every single mon in your team and can probably just lead turn 1 and win and it forces you to bring p2, metagross and probably greninja just because that one mon exists on the opponents side.
I can't say there's an easy answer to your issue and it's probably fine having that one bad matchup since venusaur isn't too common (as said above, you pretty much have to rely on hax to win), however replacing iron head is not that answer. Iron head however is pretty much mandatory and is easily metagross's best move, don't change it for anything.