Well, at the start of the game Titania is your best unit, and she's not giving that title up for a pretty huge chunk of the game. There is a lot more to what makes someone good than growths. Just having good growths doesn't suddenly make you into a top tier unit and having bad growths doesn't park you in the bottom.

First off, PoR Ike is sword locked and foot locked, and that is terrible. It's the worst weapon type with the lowest mt and no ranged options, and he can't keep up with your fliers and paladins by himself. That alone ruins a lot of his combat potential. If he's one rounding them, everyone else will be. He also happens to be in a game where axes and mounts reign supreme, which he has neither of.

So in RD Ike has bad growths, but that doesn't matter. His bases are ridiculous, 23 speed without transfers, 25 with them, he can double things forever even with that terrible growth of his, and he'll easily cap it with BEXP. A cap of 37, the second best beorc speed cap. He starts off with a fantastic sword and finally has a ranged option that he's always wanted, and he gets Ragnell extremely early. And uh, earth affinity, +45 avoid, he dodges just fine.

He also has aether in RD. It activates less, but since Ike can actually gets kills on his own and is more durable this time around, it's just extra help for when it does activate. He doesn't rely on it like PoR Ike (and also a lot of people don't even give it to Ike on PoR, wrath/resolve is better for taking down the later levels.)

In PoR Ike is just an above average sword locked lord, nothing more. Everyone just acts like he's the best, but there's nothing special about him, and the only way he\d be anywhere near the top of being the best at fighting, somehow beating Jill, Oscar, Titania, and Kieran, is if you got lucky with his levels. It's RD where he starts destroying things.