In early game Tryndemare is a beast with he´s high damage and incrediable crits, when i play Tryndemare i start out buying crit cuz´ then you will have a bigger advantage in getting the "First Blood" kill. In Late game Tryndemare makes your opponent **** in their pants, here with my build he will kill every one in a 1v1.

For Runes I go for only Attack Damage but sometimes it can be better to take only Crit runes.
You can decide what you like best.