Ran some calcs. I wanted to see if Togekiss could under any circumstances survive a +2 Attack Iron Head from Mawile (let's assume you get into a really awful scenario against a Trick Room team). It cannot, without Focus Sash:
+2 252+ Atk Huge Power Mega Mawile Iron Head vs. 252 HP / 252+ Def Babiri Berry Togekiss: 123-145 (103.3 - 121.8%) -- guaranteed OHKO
However, without Swords Dance, Bold Togekiss with Babiri Berry can survive an Iron Head even without Defense Investment.
252+ Atk Huge Power Mega Mawile Iron Head vs. 252 HP / 0+ Def Babiri Berry Togekiss: 78-93 (65.5 - 78.1%) -- guaranteed 2HKO
Sucker Punch on 252/0 Bold Togekiss only does 21-26%. So with only minor Defense investment and a Babiri berry, Togekiss can survive both. Flamethrower will always be a 2HKO, even without Special Attack investment.
Here's the other thing: at level 30, it takes a lot of EVs to make a difference in damage calcs. Test everything. This means you may be able to run well under what you think and get the same result.
Here's the upshot of everything: Bold Togekiss @ Barbiri Berry needs exactly 252 HP/52 Defense to be able to survive both Iron Head and Sucker Punch every time, save for critical hits. The next appreciable difference in calcs doesn't come in until you run 64 Defense EVs, and only increases survivability by a few percentage points.
That'll give you the win one-on-one outside of Trick Room, though it's obviously still not ideal. Use that information how you will.
+2 252+ Atk Huge Power Mega Mawile Iron Head vs. 252 HP / 252+ Def Babiri Berry Togekiss: 123-145 (103.3 - 121.8%) -- guaranteed OHKO
However, without Swords Dance, Bold Togekiss with Babiri Berry can survive an Iron Head even without Defense Investment.
252+ Atk Huge Power Mega Mawile Iron Head vs. 252 HP / 0+ Def Babiri Berry Togekiss: 78-93 (65.5 - 78.1%) -- guaranteed 2HKO
Sucker Punch on 252/0 Bold Togekiss only does 21-26%. So with only minor Defense investment and a Babiri berry, Togekiss can survive both. Flamethrower will always be a 2HKO, even without Special Attack investment.
Here's the other thing: at level 30, it takes a lot of EVs to make a difference in damage calcs. Test everything. This means you may be able to run well under what you think and get the same result.
Here's the upshot of everything: Bold Togekiss @ Barbiri Berry needs exactly 252 HP/52 Defense to be able to survive both Iron Head and Sucker Punch every time, save for critical hits. The next appreciable difference in calcs doesn't come in until you run 64 Defense EVs, and only increases survivability by a few percentage points.
That'll give you the win one-on-one outside of Trick Room, though it's obviously still not ideal. Use that information how you will.