A good Metaknight will make Ike his *****.
Metaknight has little to no lag after most of his moves which is very frustrating for Ike who's main game is punishing mistakes/missteps. Your shield needs to be up a good amount of time to deter his barrage of sword attacks - drop it too early and he'll make you pay. This means your timing has to be just right when you go for a shield grab or spot dodge. Air dodging is a huge part of this match-up especially when your making it back to the stage as Metaknight's Aerial Down A can make that a difficult endeavor.
Metaknight's UP B is a real piece of work (his loop de loop crap) as well as it has priority and great knockback range and is essentially his free ticket back to the stage. Beware of it. By the same token Ike should not play too conservative use his forward A Aerial whenever he can. Its fast and if Metaknight is at high enough percentage (thank god he's a light weight) and at the stage's edge he's a goner.
Other misc. tips..
Catch Metaknight off-guard with a counter here and there (don't spam it)
A good Metaknight is usually very offensive and will charge at Ike moreso than other characters so Ike should stop him dead in his tracks with a quickdraw when he gets the chance.
Jab, jab, jab! If Metaknight is predicatably charging Ike should spam that jab.
This matchup is tough one for Ike. It's a very steep learning curve when figuring out how to deal with a good Metaknight. Ike will fall prey to nasty combos but needs to hang in there as Metaknight's lightweight works towards Ike's advantage.