There are a few things you might want to do
I don't know what kinds of social customs the Eldar may have, but they are facing extinction. If they are ahem mechanically similar to humans, then you need to focus on the female being as productive as possible. So regardless of any life mate bonds, you need to make sure that every female becomes pregnant just as soon as she is capable of it. If the cycle is one of years, well, try to get that scheduled, so to speak so you have as little time wasted between pregnancies. You also need to make sure that the number of females pregnant every year remains relatively constant. You want this so that you don't have population waves and that as the youngsters come of age they add to the potential breeding pool at a steady stream. In addition, infertile males are yanked from the program as quick as possible. You may also want to stop exclusive pairings to add that little bit more to genetic diversity.
In addition, No more Nobility! You need to be a strict Meritocracy. Gender should not be a factor.
Women and Children First
Make the defense of the Women and Children absolutely paramount. Males can be replaced without slowing the breeding process, but females cannot. Put the women in the defensible central keep. The Men man the walls, etc. Children are protected alongside breeding age women. Older women should acutally train and study harder because they go back to the feild, so to speak, when they can no longer bear children.
Other defensive strategies
This is a bit of a hodge podge, so lets start small and work out. First, everyone learns to fight. Men, Women, doesn't matter. In fact, combat training should start young and ONLY differentiate when the young ladies can start to breed Even then, they should devote at least some time to combat training in between child rearing duties. Males to train continuously throughout their lives. This is not to say that they go full on Spartan, but things like weekly combat training classes, maybe structured like the national guard, be compulsary for everyone. That way there is still enough time to do everything else like building, agriculture, etc.
For Buildings, learn to build relatively small but strong fortified castles. When the population of a few towns gets to a certain point, a group from each joins forces to go out and found a new one. Make sure that some sort of communication link between the castles is an inherent part of the construction of each colony. This makes sure that if one is under attack, they can get help from nearby castles. it also avoids the whole "eggs in one basket". If plague hits one castle, you can quarantine it. If there is an invading force, they would have to stop to lay seige to a series of smaller fortresses, which is very costly and time consuming.
Depending on the Eldar's numbers, this may just be whistling past the graveyard. It may just be delaying the inevitable, but it might work