How do I beat shandor,ascendant?

Do you mean when he is in normal form or when he is in his destructor form?

Shandor Ascendant is one of the hardest characters in this game to beat, that is for sure. Before you can get hits in on him you have to slime him for quite a while. It may seem like you're not making any progress but you are and eventually when you switch to the blast stream you'll be able to give him a downward pull and get him out from behind that skull mask. If you find that slime blower is taking a long time to loosen him up, switch to the blast stream and shoot him with that along with a couple of boson darts, I find that sometimes helps the process along. When he gets in that force field with the two stone pillars beside him, switch to your stasis stream to freeze the pillars and them blast stream them away. When Shandor does eventually pop out from behind the skull mask, hit him with the blast stream and boson darts as much as you can in what little time you have. The last hit to finally kill him does take the longest, so just be patient, stay alive, and mix things up from time to time if you find things aren't working well.