You may have to deal with a defensive Metaknight, one that is unwilling to go into the air, and thanks to possible in his percent / stock lead, justified in doing so. At this point your options are limited, you can attempt to drift in and pound or aerial through his tilts, or you can stick to the ground and dash grab. The latter option can be countered by MK's dtilt, unless of course you power shield the hit and shield grab him. It is your job to always try and push MK up into the sky, that's where we will win this battle.
As far as killing goes, we actually have the advantage in this factor. Not counting Rest or Rollout, the former can be too risky in competitive play, and the latter will be punished by MK, our best options are dash attack, sweetspot fair, and utilt. Fair will likely be your most common kill in this, and you can position a Metaknight into it if you grab release him off the level. Do not stale this move in this matchup. MK's best kill option will be Dsmash and likely nair. Work on avoiding those moves, the former by staying the air as much as possible, and the latter by maintaining your aerial spacing.
Now, Jigglypuff also has a hilarious trick up her sleeve in this matchup -- We can rest Metaknight out of his tornado. Practice this, as it effectively limits the use of one of MK's best moves. This can net you early kills, or even just a chunk of damage. Just be prepared for the consequence if it doesn't kill him, and you're vulnerable. For this reason, I tend not to rest through the tornado until I feel it is strategically important to do so, when I know it will net me a kill. On this note, if you need to beat the tornado, there are a couple of options. First is Dtilt. Yes, THAT dtilt, the move without a use, has a use. It's rare that you will have the opportunity to use it, but be wary of your options. Secondly, nair. Sweetspot that nair and you'll beat the tornado from any angle. Thirdly is dair, pop up out of the nado if you're caught and immediately dair to beat it. Finally, pound, the move that beats almost everything. If you need to beat Shuttle Loop, dair is usually your best bet.
Overall, I feel this is in Metaknights favor. This is no surprise, however I feel that we are better equipped to deal with Metaknight than most other characters. For this reason, if I had to give a ratio, I would say it ranges between 55-45 to 60-40 Metaknights favor.