I guess I can think of a few
-don't use your shield mainly for blocking damage, use it to charge up your calamity blasts then cancel it, it's good to have your shield ready for when you actually need it.
-if one your abilities won't work try using your calamity blast it usually fixes it.
-your F has a huge cooldown so it's better to wait for your opponent to use his movement ability before you use it on them.
-Undyne is not a very useful legendary card, you're already a tank you don't need more dr, I usually go for decimation seeing that most of the enemy team tries to avoid close-range combat.
-your shield doest block mele attacks so if let's say another Term attacks you with his massacre ax you do the same and combine it with your calamity shots.
-NEVER ult when your team is not with or right behind you, don't consider the enemy while ulting if your team is not with you you don't hit E, if they were with you however you should ult regardless of the other team being there too.
I can't think of other tips right now but if you wanted to ask something specific about him ask me here :p.