The only thing you are doing wrong is dying. You need to defeat the assassins and then track down Yarun again (note where he is before he disappears). What level are you at? I don't recall the initial assassins being that tough, but if you just made it to Verdistis and have not cleared much of Rivertown, that may account for the difficulty. In that case you can put off finishing this quest until you level a bit and/or get better equipment.
If you know where the assassins are coming from, you may be able to position yourself better before talking to Yarun.
Aura of Guarding and Spell Shield will give a little extra protection for physical and magical/poison damage, respectively. Otherwise, more
restoration potions should help.
Also, check your resistances. I don't think the assassins use elemental or magical attacks, but they may use poison.
The techniques in the following topic will work against and strong opponents, though it contains minor spoilers.
Chasing Josie... techniques for defeating Josephine