Imagawa led a force of about 25,000 men, where as Oda Nobunaga emerged victorious with only about 2-3,000. This victory catapulted Oda into historical significance. From what I understand, Imagawa never saw him coming. Oda used his knowledge of the terrain and information from his scouts to perform a sneak attack on his enemy's main camp, resulting in Imagawa's death. As said in the topic, can this be considered what we would call today, "Guerrilla Tactics"? This is my first post here, please be gentle if this question is silly. Sengoku Jidai is one of my favorite periods in history.
I think you are having a misconception about "Guerrilla tactics". The term as we normaly use describes a form of warefare that a small group of personel (usually irregulars) using ambush, sabotage, hit and run... to attack a larger enemy force. Guerrilla tactics are basically harassment, aim to tie up or to bleed enemy slowly. If Oda sent, like, a hundred of guys attacking Imagawa men everytime they were out getting water all the way from Mikawa to Owari, then it's guerrila tactics. But his plan in Okehazama is a surprise attack, using an organized army with the goal of a decisive victory in mind.
So no, it was a sneak move, but it was not Guerrilla.