IC is a pretty easy match IFF you don't get grabbed. To not get grabbed you will have to be extremely unpredictable, and approaching with spaced NAirs works pretty well because if you miss you can still jab to stop a grab. If IC is approaching you in a dash, you can catch them with a Falcon Kick or a Raptor Boost unless they are expecting it and shield it, at which point you will get grabbed. IC's dashing shield is their best approach, because if you attack them with anything you will get shield-grabbed. However, most ICs will just whittle away at your health with Ice Blocks and Frost Breath when you get close (which I havn't found a way to get around yet, since it isn't angled down like Fire Breath). If you have your Knee timing down, you can let them approach you by standing outside of Frost Breath range, and if they decide to Ice Block you instead of approaching you can SH Knee them.
In general, Falcon Punch works pretty well if you surprise them with it, as Nana often lags behind Popo and won't shield/roll/dodge in time, and even if you don't hit both, by himself Popo pretty much just has mindgames since his range and power isn't that good. Just be warned that if it doesn't hit either of them you will get grabbed. If you get Nana separated from Popo but not killed, chase her and knock her offstage. Popo will also often chase her to try to save her with their up-B, so it can be a good time to kill him too, or if you knock Nana out farther Popo can't keep chasing because he won't be able to get them back on the stage (if he tries, she will die before he can get in range of her to up-B). Edgegaurd if he uses his second jump to try and save her. When Nana is trying to recover from above the stage, she likes to shoot Ice Blocks on her way down, which makes her hard to Falcon Punch because it pauses her fall. You need to get above her to hit her with it, or just go for a Knee, UAir (hit with her in front of you), NAir, or Falcon Dive. Raptor Boost might work too, but that is pretty risky. If you play it smart, once Nana is dead you shouldn't take too much more damage (if any) before killing Popo, since you can outrange him pretty easily.
