Hmm, let me help with that a little.
1. If you can, chase down wounded aliens for the kill. Letting those dudes sit around and heal means that they'll come around again soon. If your team uses teamwork and is more aggressive, you can chase those aliens off of your doormat. If you are wounded, you can retreat and either use your med-kit or walk up to a medistation.
2. They have tons of evolve points because your team died too much. Try not to feed. Teammates don't let teammates feed. When you leave the base, go out as a team and you will survive as a team.
3. Dretches are speedy and small, thats who they are. But they are also fragile, and will die quickly. If you can see them first and hit em, you probably won't have a problem. Only 5 rifle or chaingun hits will kill, 3 lasgun or pulse, 2 flamers, 1 massdriver, shotgun 1-2 shots, and 1 luci spash or mini-luci hit.