Catholicism is known for being opposed to euthanasia, even voluntary euthanasia. What would be a plausible penance for such a sin though?

Let's set some ground rules. There is no doubt in a given instance that this is voluntary and that survival is not feasible, and the decision must be taken quickly. Perhaps a platoon of soldiers in a war where one of them receives a wound that is impossible to survive, though not fatal instantly, or where there is an essentially 0% chance of survival even with medical care or medical care is inaccessible. They already know among the platoon of the intention of being part of this pact. Perhaps one of the soldiers after the war has carried it out, and this along with the general trauma of the war makes them turn to a religion, in this case Catholicism. There is an amnesty in the peace terms so civil prosecution is not possible. Sins are normal, in fact, something that all the humans besides Jesus (and Mary to Catholics), have done, and penance is necessary for all Catholics. Priests deal with common sins all the time, but getting this kind of confession and request for a penance prescription would be quite atypical.