Aglar is a test of your Brutalize ability, a special kill which requires striking from stealth. Your optional objectives are to carry off the kill without killing any other orcs, and to do so very quickly. Luckily, these objectives all complement each other nicely.
Start by hurtling yourself off to tower to the right . You'll want to hug the right edge of the map here as much as possible.
Jump up to the parapet and scuttle along the balustrade. You can actually break into a sprint here and not raise the alarm.
Do drop into stealth for your final approach on the target. Remember that Brutalize only requires a tap to initiate- hold down the button and you'll accidentally find yourself in bow mode.
Execute the attack to end the mission.