Saucers are one of the hardest aliens in the game. They are extremely large and fire powerful shots. They do not move very fast so a faster ship is better for killing them as you can trance around them. Saucers also have extreme hit-points. Yellow Saucers have 1000 hit-points red Saucers have 2500 and purple have 4000. These are bosses and therefore only spawn on certain levels. Yellow saucers spawn only in wave seven, while red saucers spawn in waves six and ten. Red saucers spawn in waves 6 and sometimes in 10 with the final boss. They are quite dangerous so it is recommended to kill them in groups of three or more. Some saucers give loads of gems, for example, yellow saucers give almost 300 gems. If you are around a saucer try for it but if you are closer than a teammate it will chose you as a target. For more on saucers go to https://starblastio.gamepedia.com/Alien.