Yeah the one with the large blast was indeed a big battlesuit, unsure of what kind it actually was but it was either AP3 or AP2 Large Blast because it removed the armor saves of my marines which is 3.
It was a riptide with ion accelerator then, thats in the Tau Empire codex.
This was a combined arms detachment and the drone formation. I'm very leary on this one because crisis suits get expensive fast.
How many crisis suits were there in total? What kind of weapons did they have?
Even if I assume the second squad was only 2 strong, and all suits had plasma rifles and missile pods his list would have been over 1000 points.
This is the Grey Knight list based on what you said, theres likely something missing but regardless this would be extremely tough for your deathwatch to deal with. It can get in your face very quickly and dominate in the psychic phase with 9-14 warp charges per turn.
So of the three lists you faced, Genestealer Cult is tough for everyone in general, the Grey Knight list was the rock to your scissor, and that Tau list might not have been legal or else it was super competitive.