Backstory: One space-faring civilization detects evidence of intelligent/sapient life on another planet. They send emissaries. The people on the planet, a species of intelligent apex predators, promptly murder the delegates, take apart, analyze, and start producing crude copies of their ships, and begin prowling the universe as murderous space pirates hunting the other species for sport.
In order to explain why a somewhat less technologically advanced species from a single planet can strike terror into the hearts of a more advanced, more populous people, said alien species has a rather impressive list of abilities, as follows. (I'm being quite detailed here so I can refer back to this post in any further questions about this species, rather than typing it out again, though most of it is pertinent to this question as well, so I would recommend reading it before answering. The specific question is in the last paragraph.)
- Great physical prowess: they are extremely fast and strong (fast enough to catch an arrow or deflect bullets, but not the Flash. Strong enough to rip off your arms, but not Superman). Essentially, their skeletal and muscular structures are much stronger and more efficient than Human Joe's.
- Enhanced senses: they have excellent senses of smell and hearing (they can track a person like a hunting dog if need be), their eyesight is approximately on par with your average human's, except their night vision, which is vastly superior. They also have the additional sense of heat detection.
- Virtually impossible to kill: their bodies heal at an extraordinary rate. Wounds that would be fatal to Human Joe due to blood loss or irreparable damage aren't necessarily fatal to them. The best ways to kill them: Cut off the head (and make sure it stays off long enough for them to die), rapidly destroy such a large portion of the body that it is impossible to regenerate before death ensues, or continue damaging them continuously over a long period of time until they "run out of juice" and can no longer heal themselves.
- Shapeshifting. The species can shapeshift, with some limitations. They have to turn into something alive (usually something real they have come into contact with before, though small details beyond what species they choose, such as bone structure or hair/fur color, they can generally use their imagination for as long as it doesn't break the functionality of their basic chosen template). They must stay within a certain margin of size in accordance with their original mass (they can make themselves bigger or smaller to a degree, but only have so much overall mass to stretch or condense). They cannot turn their bodies into non-organic tissue (aka no metal skin. Bone or scales, perhaps, but not metal).
There are some materials inherent to the species' homeworld that are "compatible" with their shapeshifting, for instance, the hides of certain animals and the fibers of a certain plant. They use these to make clothing and can incorporate them into their bodies when shapeshifting if they desire. There is also a kind of metal that they can change the form of, but not to the extent they can the compatible organic materials. Aka, if you are wearing a leather coat and a metal sword, and turn into some kind of animal, the leather may become part of your animal body, but the metal may not. You can, however, change the sword's form into something more convenient for your shape, such as a metal plate on your back. This metal is very strong, very heavy (by Human Joe's standards), and very expensive due to a difficult mining and forging process. In addition, your ability to affect compatible materials is limited to things you are touching, and to a certain range away from yourself (say, about six inches). So, if you are holding your metal sword, you can reshape the handle and a few inches up the blade away from it, but nothing beyond this. If you are only 3 inches from your sword but not touching it, you cannot alter its form.
The species are fearsome fighters, and in any kind of close quarters battle they can take out large numbers of Human Joe type people. Normal handheld projectile weapons aren't terribly effective against them, as they a) are difficult to hit b) bullets/small projectiles don't usually do enough damage to kill or greatly impair them and c) they tend to close range very rapidly and you only have so long to shoot at them until they are on top of you, especially in cramped quarters like spaceship corridors. I'm not going to go into detail of what kind of weapons people devise to use against them since that isn't the point of this post. I might make another post on this topic later.
Here's the problem: none of their advantages are of much use in space piracy and murder until they are actually on board the other ship and can rampage to their hearts' desire. So, here is my question: You are a murderous, virtually unkillable alien species who wants to go slaughter all the people on another spaceship; given the above described abilities, how do you get off your pirate tub and over onto the poor hapless victim's ship? You don't target only unarmed ships, you welcome the challenge of attacking ships with weapons also, if you think you can get away with it.
I almost didn't post this question for fear it was too subjective, but I think there are only so many possible answers that would actually practically work, since it isn't necessarily easy to board a spaceship against the owner's will, especially an armed one, and I'd like to know if there are any I've overlooked. There may be some options open to these aliens due to their survivability that aren't open to Human Joe.
NOTE: Someone might bring this up, so I'm adding beforehand, they can indeed survive exposed in vacuum for a period of time, though probably not indefinitely.
Edit: In response to some comments, yes, this species does indeed happily attack colonies and slaughter the people on them. I didn't mention it simply because it wasn't part of the issue at hand. They hunt the other intelligent species partly because it is more of a "challenge" than hunting animals, and the more challenging a target you successfully attack, the more "sporting" it is. Ships are harder to attack and locate, which presents a lovely little challenge to our predators.
Some of you are also asking about infiltration or trickery. This is definitely possible, though limited by the fact that most of the predator race don't speak the other race's language. Also, some of these methods would be initially handy, but as the predators gain notoriety and essentially became like the boogeyman, those same methods would become less effective because the victim race would have become wise to them. Other methods, like the distress beacon trap mentioned in one of the comments, would probably still be useful, but harder to pull off.