Proceed with the final leg of The Assassins Of Kings only after you have completed all of the quests, side quests, and contracts you want to do in Flotsam. After completing this quest, the future of Flotsam will be altered in some permanent way no matter which decisions you make, and you may not be able to complete any outstanding missions you’ve yet to undertake.
That said, head out to the elven ruins where you picked the Rose of Remembrance to find Letho. Along the way you’ll bump into Iorveth, who will suggest that you take him into the ruins as a “prisoner” in order to fool Letho into letting his guard down. Upon arriving at the ruins you will meet Letho and learn more about his grand plan. You will learn of other kingslayers, and Letho will hint that he knows Geralt somehow, although with his fractured memory Geralt cannot confirm this. In the middle of your conversation, Roche will storm the encampment with his men. Iorveth asks for a weapon.
Your decision here has far-reaching consequences, so think carefully before you act. If you hand a sword to Iorveth he will immediately turn on Roche, causing the town of Flotsam to revolt against non-humans. Elves and dwarves will fight to the death in the streets as the city burns in chaos around them. If, on the other hand, you deny Iorveth the sword, Geralt will knock him out and Roche’s men will capture him. A feast will be thrown to celebrate the capture of the notorious criminal. One of these options is naturally preferable to the other, although players with a chaotic streak may wish to hand Iorveth the sword just to watch the chaos that ensues.
As the Scoia’tael and Roche’s Blue Stripes clash in the clearing, Letho will lead you into battle in the elven bath house. Letho is by far the toughest opponent you will have faced so far, and as such you should take extreme caution when facing him. As a Witcher, he possesses many of the same powers as you. He will cast Igni to attack you from afar, and he will also throw poisonous bombs across the room at you. As soon as you see him winding up to throw a bomb or cast a spell, roll to the side to avoid the attack. Letho also casts Quen on himself frequently, shielding himself from damage. Occasionally, if he gets close enough, he will cast Aard to knock you off your feet.
At first you should focus simply on staying alive. Learn Letho’s attacks and how to dodge each of them before you commit to any serious counterattack. Use Quen when he launches any particularly serious attacks in your direction, and spam Aard to keep him at bay when he comes in for a close attack. Once you have familiarized yourself with Letho’s attack patterns you can successfully mount a counterattack. Set Yrden traps across the battlefield. If they succeed, Letho will be momentarily stunned and he will be unable to counter any damage with his Quen spell. Don’t waste any time with weak blows; use the right mouse button to hit him with heavy attacks while you still can.
Other traps can prove helpful in this battle as well. If you have any incendiary traps, set them up towards the middle of the arena and attempt to lure Letho into them. When he trips the trap, attack him with heavy blows as he burns.
Although Letho’s health bar may look intimidating, rest assured that you don’t have to drain it all in this battle. Once it gets about halfway down a cutscene will trigger. Both Witchers will cast Aard, but Letho will eventually overpower you. He disarms Geralt but refuses to kill him, claiming that Geralt saved him once in the past. As he leaves, he informs you of his plan to capture Triss and force her to teleport him away from this hostile area. Be sure to pick up your steel sword before leaving the area.
Either Iorveth or Roche will come to help you, depending on who you helped. If you helped Roche he will accompany you into Flotsam to help search for Triss, but if you chose Iorveth he will leave you in the forest because of the non-human massacre occurring in the city. Thus concludes The Assassins Of Kings and begins the next quest, Where Is Triss Merigold?