don't try to repeatedly stun the enemy because the enemy can be stunned only every 3 second(unless you're using the wekono's wrath legendary) and that goes for every enemy. otherwise it's a good plan to use your stuns in 1v1
ruckus doesn't have a shield in front of him that protects from stuns, so use your reload to deal with him. just avoid his high burst damage before you stun him, after that he has to spin his weapon for a second to get it back again.
as a mal'damba you are no match for koga. try to be on lookout for him constantly watching the flank route and if you see him run away. try to get to your teammates for help slithering to the tank is a good idea if he's not busy.