We dropped into the depths of the Hive World Meridian, to kill a Tyranid Lichtor. Waiting for us was not only the Lichtor, but numerous support structures. First order of business was to secure an Imperial Shrine, located unfortunately far away from the rest of the action. Didn't stop the 'nids from putting up a hella defense of the Shrine.
Next, I hit one of their support bases, carrying a full-on hive and several synapse nodes. Lost several Marines to Spore Mines. Took several minutes of sustained fire to take out the hive. The Lichtor rushes my force immediately after that, along with more, lesser, Tyranids. The Lichtor takes out most of my force, the other take out most of the rest, but it dies. I regroup and reinforce, then press on to the other bases. I lose more Marines to Spore Mines.
No more hives, although one base had two of those things that spawn Spore Mines. Tarkus shoved a grenade down one. That worked. Another grenade one-shotted a Hive node(which shoots Spore Mines).
I think I lost more Marines on this battle than all previous battles combined. My Force Commander was knocked out at least three times. What the hell are we, the Imperial Guard?
Even so, I won, and collected a Prime Tyranid Gene sample, and reduced the Tyranid infestation on Meridian.