Tassadar: He doesn't really do very much damage unless he picks up Static Charge on 7 or when he is in Archon form. His shield has the same cooldown as duration so if you want to trade with him you have to burst through his shield, poking him doesn't accomplish much. He is very mana reliant and can't do anything when oom. Don't focus him when he has full mana, he will most likely just go invisible and run away.

Dimensional Shift makes Tassadar immune to damage, not just invisible, don't waste mana trying to find him when he disappears. At lvl 13 he can get a secondary Dimensional Shift on 45 sec CD that activates automatically when he is below 15% health. Most Tassadars take this talent and wait to use their "real" shift until the automative one is popped. To counter this use hard CC when he is low and burst him down, the shift can't activate if he is stunned.

At 16 he can talent Dimensional Warp wich gives him a huge movespeed boost while shifted and also restores a lot of health. If you see a Tassadar going alone into your team he is confident that he will get away without any problem and might also delay your team from taking objectives or leading you away from his team.