I don't play metaknight but I do have some decent experience playing against IC's. Some general stuff to know.
If the IC's send out an ice block and chases it with popo, just jump and land on a platform. It's a free grab for them if it touches you or if it gets shielded because of the shield/hit stun.
You want to separate the two IC's. It's pretty obvious, but it's important enough that it should be your entire game plan. Every time it happens, you kill their desync game, their grab game, deal 30-50 damage to 1 or both IC's and get a chance to gimp nana. This should be your goal.
Playing a ground game is riskier but very rewarding. Most characters have better options to separate IC's on while on/the ground. One IC's player I know spends the whole game trying to keep his opponents on platforms/air camping for this reason.
Grabs are good against IC's. It's a guaranteed way to separate the two. If you get the oppotunity to get one, do so. More importantly they can't CG out of a throw which is one of the reasons it's so effective. Just try to make sure that it's an easy punish or a trustworthy read because you do have to get close to use it.
Don't let IC's set up a desync game. You'll never gain anything from it. Unless you're planning on timing them out, keep a constant threat on them so it's not an option.
Sharking is extremely good against IC's do it you ever get the chance. It's not like they can really even do anything to you. The only thing that hits significantly beneath them is Dair, and I can't even begin to describe just how bad it is in this game.
Counterpick to brinstar >> Rainbow Cruise >> Delfino
Brinstar is terrible beyond belief for IC's, expecially ones that like to play desync'd. You can shark easily. The little attatchment in the middle makes it hard for IC's to desync and absorbs ice blocks. The side platforms aren't convenient to Uair. The stage is just so bad for them. The slanted ground on brinstar also changes the timing for the chaingrab so it's slightly more difficult to do. Oh, and I almost forgot about the acid interfering with the chaingrab. Believe it or not, the earlier things are more relevant.
Anyway, Cruise is the next best thing. IC's are forced to move around and that puts them in the air. This shuts off a lot of desyncing and chaingrab opportunitys. You can shark here too.
If both of those options are shot off for whatever reason, Delfino is a good option. There are lots of opportunitys for sharking. There are lot's of split levels. If you have the high ground, IC's can't sit at a distance and send out ice blocks and are forced to get in the air. On any part of the stage with water. Just run straight into the water and go off far enough that IC's would have to burn their second jump to chase you. At this point, there's litlerally nothing IC's can do to punish you. Using their second jump kills all of their options. At this point, it you are in the water, they have to land in the water to hit you. Just avoid it, wait for them to land and punish. You can also just get back to the stage, and start taking cheap shots at them while they try to get back. When the stage transition comes up, you can just wait on them to get up because they don't have any attack to fire off and still be able to make it back to the stage. It's great for burning out the clock.
IC's want to counterpick to FD then smashville in that order. Ban FD and practice against IC's on smashville when you get the chance.
Playing with IC's is extremely stressful as is playing agaisnt them. There's so many things that can go wrong. Separations, dropped grabs, the difficulty of desyncs and chaingrabs, nana being dumb, the power of counterpicks against IC's and so on and so forth, can really work a number on IC's psychologically. If your can get in their head make them impatient, etc. You stand a good chance of winning the matchup.
The converse is also very true. I've seen good players, and players that know the matchup well, punt it because they can't keep their head on straight. Don't let your self think outside of the game. The methods will differ from person to person. If you have something you do/use to calm your nerves before playing IC's do so. Repeat afterwards as well, you can't let a loss, or how you barely won affect you next round.
When you get grabbed, you need some way to calm down. This is where the stress levels are highest in the match. I've seen it happen time and time again, someone is sitting next to the IC's player and raging so hard that he gets out of the chaingrab only to get regrabbed half a second later because he's too mad to tell the difference between a jump button and an attack button. Anyway before you start, make sure you know what this is. Personally, I view it as a time to rest my brain and space out. View it as the time between kicking a field goal and setting up another kickoff in football. It's a chance to relax.
One last thing: all IC's drop grabs. I'm not kidding when I say this, expect the chaingrab to be dropped. If you don't believe me, go watch the higher level players. It happens to them with more frequency than you'd think.