(Also what Tau weapon is a large blast, and is ap3?)
At least AP3 you mean. Might've been a Hammerhead with an Ion Cannon, but more likely it was the other big battlesuit mentioned - a Riptide's Ion Accelerator seems a likely candidate. Or he might have just gotten unlucky on the saves...
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.