Aesir vs Vanir war - Counter to Thor

So I was just thinking after finishing new game plus once again. Freya says that they held back the invasion of the Aesir, basically the most they could achieve was a stalemate. But even at Ragnarok, it comes off as if the only chance to beat Thor was Kratos and Kratos alone since they knew that even the world serpent would be simply sent back in time.

So my question is how were the Vanir able to hold out against the Aesir? Beating Thor or Heimdall seems impossible to anyone not called Kratos. Are there any answers or implications to this in the game that I don't know about? Or am I just going way too deep into this theory for no reason.