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...
Ah your right, most likely was the ion accelerator.
I'm rather curious to see the Tau and Grey Knight lists to be perfectly honest. The Tau list is rocking 415 points between the normal and Y'vahra riptides with only the ion accelerator upgrade, and the Grey Knight list is at 425 between a stock terminator squad and 2 dreadknights (535 when including the librarian with no upgrades.)
For your Tau opponent, its more a matter of wondering if he was fielding the correct number of points. I mean, between the two riptides, the compulsary crisis team, a stock strike squad, and a cadre fireblade your looking at 604 points. Thats 600 points with no upgrades except the ion accelerator, and no markerlights yet.