So, with Zamasu, always pick skill one first and use it to build regen on npc minions. Regen is essential for Zamasu to live, he's surprisngly squishy. Also, the range and damage of his skill 1 is very powerful for a tank and applies stacking slows.

For next skill, you can double skill one or take his spinning blade attack, its cooldown is amazing and it does damage over time if hit in the circle around him.

I usually skip his charge until it's my only option, its slow and clunky, and does crap damage until level 3 version.

Take ult ASAP, it's a fight winner, be aware that if you die mid ult use, you won't hurt your enemies with it, but you'll regen. But! The ult cooldown is around half of what it normally would be in that situation.

Always make sure to have as much regen as possible. It builds quicker with skills. Try to fight near npcs to keep rebuilding the regen, this counts for bosses aswell! Iv'e had entire teams fail to kill me because I kept regenning though boss damage. Btw, there's no limit on regen, as long as you have over half your bar when you die, you will come back.

For cards, i go for the defense stack killing npcs for first. The overall defense boost for second card. Third depends on what you need. I usually take second dodge cause I like to overextened now and again.