To Interrogate penguin’s arms dealer you will need to defeat 8 enemies in one fight. To do this without having to fight anyone and not loose health you will need smoke grenades and batarangs.
Start off by throwing a guided batarang at the enemy with the gun. This will make him drop it and make things easier. Now use the other batarangs to separate one of the enemies from the rest. Use the guided batarangs to do this in a precise manner. Once you have separated a person throw a smoke grenade just behind them and drop attack onto them. This will push them into the smoke so as you can quickly eliminate them using the ground takeout move. Once the enemy is dead imediatly grapple yourself out of the fight.
Now that you are on the roof use you have to wait until your smoke grenades are ready to use again. Whilst the grenades are on cool down use the batarangs to separate another enemy from the group. Once the smoke grenades are ready to be used again throw one behind the enemy that is separated and take him down using the ground takeout, then proceed to get off the ground using the grapple.
Keep doing this until there is only one enemy left. Defeat the last enemy and get the information to complete this part of the mission.