some strategies
1.use the bomb alien and place it in large cluster of enemies this will clear most of em out
2.place eggs in areas far from enemies or in areas cleared of enemies
3.place the demolition alien (the alien with large claws) after your grunts have cleared the area they will do damagae while your grunts keep the enemies at bay
4.do previous levels to get more evolution points to upgrade your aliens and buy new ones!!!
hope these strategies help.......