Got my first dose of DA->Uair string->UpB at a tourney this past weekend and tbh, I don't ever want another. How do you deal with this string as floaty and heavy characters? It seemed to work at every percentage, on both of my characters (Rosalina and Mega Man)
Twiranitar gave a good answer to your question but I will further elaborate on it.
Assuming the Meta Knight knows the general range of when he can Death Combo (it is a combo not a string when done correctly fyi) and can account for other factors such as Rage and how stale his Up Airs are, your only shot is to hope you can DI and throw him off. But here's the caveat:
There are two kinds of Meta Knight players who can Death Combo: The ones who understand the inputs and the ones who understand the nuances of the combo as a whole.
The former tend to just start their Death Combos and have it fixed in their head that the opponent is only going to stay in a certain position (probably from playing against lesser experienced opponents/computers). As a result, these Meta Knights often drop combos to players who understand what DI does here because the Meta Knight will realize at the last second, "Oh **** he's too close to me!" and then drop the combo.
The latter is what you need to be concerned about. These Meta Knights react to your DI on the fly and have enough practice and hand dexterity to adjust mid combo and make sure they are in a position to finish you.
Unfortunately, Rosalina just gets bodied by Meta Knight to begin with and the Death Combo is the icing on the cake. Megaman does better but there's a rather large window that combo works on him due to his weight and fall speed.
It really is a matter of trying to mix DI to throw off your opponent assuming the conditions have been met in order for it to kill you.