Okay, if your guys are going to burn down genestealer stats anyway, take another approach.
The Ordo Xenos has been observing a new and potentially devastating tyranid xeno-form originating on the outskirts of Hive Fleet Dagon. Reports from survivors of the attacks of this breed indicate that they are an infiltration organism that are capable of independant breeding. A secondary organism deposits a fist sized seed pod into the vitals of the victim and disorientates the victims mind, making them largely forget the incident. Between a day and a week later, the gestating creature has absorbed enough nutrients to burst forth in a spray of gore, and begin to hunt non-tyranid creatures around it.
The civilian void station, Vendegrath Orbital, was towed into position around a gas giant where its crew were informed they would be siphoning gaseous promethium for the crusade efforts. The location was secretly selected by the Ordo as a prime hunting ground for this new xenos threat. Within four months of their deployment, the crew of Vendigrath Orbital granted clearance to a inter-system garbage scow to dock. One of the scows crew was infected with the organism which subsequently gestated, burst free, and infected the Vendegrath crew before eventually being destroyed by a vengeful crew armed with refining lasers. Fortunately, the infection was spread, and the Ordos now have twenty field subjects to experiment on to discover the weakness of this news species.
The Deathwatch have been summoned, as the creatures are lethal to humans within moments of their eruption. Moreover, the Imperial Guard would be ill suited to dealing with the creatures - they would be too likely to kill them, rather than contain them should the unthinkable happen and one of the victims erupt before they are escored to the cyrogenic pods on board the astartes vessel.
The mission is a simple one. Escort twenty prisoners across a space station to cyrogenic pods. if any of them don't make it, capture (not kill) the greatures and store THEM in cyrogenics pods. They're not a threat to the astartes of the Deathwatch, but their acid will certainly damage the armour. if your players just kill them outright, the Inquisition will start stripping renown like it's going out of fashion. You can time the chest bursters to pop out whenever you want to - probably at really unpleasant moments. if your players insist on terminator armour all the time, this mission will punish their slow moving asses.