Okay , but slaansh are very are very fast (they can run and after that charge) so I will not be able to keep him at bay for a long time, maybe one round. but I will test your tactics, but I do not have 20 chameleons, i have only 5 but i have 6 x Rippers okay but i have only 2 bastiladons
Stagger your skinks like this: This way it takes 1 turn to reach the first unit of skinks, at least 2 turns to reach the second, and at least 3 turns before they reach the bastiladon. Possibily more if you get to use wary fighter or get Lucky and your skinks actually survive a round of combat. Bastiladons and chameleons are by far the most effective thanks to their bonusses and superiour range, but anything that can shoot will be a fairly significant boon against slaanesh given it's complete lack of ranged firepower and how squishy they are. Razordons & salamanders will put out quite some damage as well when placed behind a skink screening force. Razordons especially can be briljant as you first get a round of shooting as Slaanesh is stuck on the skinks and then a round of overwatch fire with -1 rend as they charge into the razordons. Which is gonna hurt at 2d6 shots/razordon. A stegadon with sunfire throwers will also be able to put out quite some damage on one of his daemonette units if you can Ensure it goes first or gets some rounds of shooting in protected again by skinks. O, and lastly there's Always Kroak. Surround him with skinks and proceed to blast any daemons apart with his comets & celestial deliverance while summoning if he doesn't dare to come close.
Do you play for objectivs? Because the picture seem to show you meeting him half way on the board. For me the Chameleon Skinks work like a charm. As he is a friend, Just proxy some for a battle.