Should I go for single element team, or all poison, all sleep, all stun team? Or should I mix with different elements and different strategies?
Well, you can use a single element team, a sleep/stun/poison focused team, but mixing strategies, combos, pairing powerful monsters in a good synergy is the key to the victory.
Apart from link skills there’s not a huge amount of benefit going for one element.
Personally I like mixing it up with different strategies and I encourage you to do the same. If you run stun or sleep and hit a load of immunes you’re going to struggle. It’s good to counter your counter or have a back up plan. Poison is probably an exception where full poison teams can work pretty well.
Mix and match, but not too much that you need the stars to align to work. I see players running some link mons where only 3 or 4 monsters are the correct element and that’s never gona happen against good players.
Mix the elements, mix the strategies up a bit. Don’t run poison and sleep together obvs.
Agree with this comment. Good luck!
I was waiting for the experts to give him a better advice, thank you! 
I’m no expert. I just like to give advice where I can 
use AP spam
(Another cute and meowful pic, love it!)