I had to do a lot of runs for a second win against Iron Leaves.
The main problem is that it needs very specific coordinated play to beat.
I've tried a support Armarouge with Will-o-wisp and Reflect. In my final run, I had to resort to using Taunt so it doesn't get Sword Dances off.
But it's like super complicated. One way to deal with the terrain is using another but... Grass and Psychic boost its STABs so it's not a good trade off. And most Ice Spinner users are weak to either Leaf Blade, Psyblade or Megahorn.
And with the dumb shields and status wipe it can heal off burn and with shields up, we can try it again unless relying on stuff like Flamethrower
I was looking at bulbapedia and a support Scizor with Defog might be a good bet since it resists the 3 main attack moves and Defog gets rid of terrain (and the Quark Drive boost) but I'd say an ally with taunt, reflect and will-o-wisp would still be the most optimal.
Forretress:
Level:100
Nature: Adament(Attack+,Sp.Attack-)
Ability: Sturdy
Terra type: Bug
Max EV’s in Attack, and HP
Held Item: Leftovers
Move set:
Reflect
Ice Spinner
Curse
Bug Bite
Now that's an interesting set. Ice Spinner gets rid of terrain and Reflect boost defense.
Shame it doesn't learn leech life for sustaining.
But looking at the moveset, Payback seems like a good way to attack it since it doubles base power from going last since it's so slow.