tips for defense missions plx
co-op is always useful for those missions.
Infestation defenses are usually a little easier at first as there are fewer ranged infested units.
Also, try and get melee units (particularly grineer/corpus) that are attacking the target really quickly, as they do a surprisingly high amount of damage on it, I find.
Make good use of your abilities too. You're playing as Mag, right? So your 4th ability is really good at clearing up loads of enemies all at once. If you team up with a similarly leveled excalibur their 4th ability also lets them cut down crowds really quickly.
Having lots of enemies swarming all at once also means you should get pretty frequent energy drops close by, so you can keep using your abilities.