|She wears WHAT?!|
1. You Really Need To Watch Your Tone
Not swearing - go ahead and fucking swear if you want, you fuck. I'm talking about the overall feel of your work, or your voice if you like. It's important for a single book to have a consistent tone throughout, or to have a tone that evolves in a natural progression from beginning to end. What you don't want is to ping-pong between like four different tones.
Suicide Squad ping-pongs between like four different tones. It opens in Gauntanamo Bay, Louisiana, then jumps to a bunch of flashbacks that go tense-political/standard-supervillainics/politics-again/WTF-Harley/more-politics/horror-movie/back-to-politics, and that's just like the first ten minutes! The soundtrack doesn't help because none of the songs stay on for any length of time - I'm not sure there even was a score, it was so lost in the rest.
If the movie had maintained a consistent tone (preferably not the darkest one), it could have been a hell of a lot better. So take a look at what you're writing/creating and make sure you're not bouncing around like Harley on a surprise meth injection.
2. Don't Spare The Knife
Task Force X, the titular Suicide Squad, is a gang of criminals that get picked to do dirty jobs for the U.S. Government in return for reduced prison sentences. That's it. It's the Dirty Dozen with supervillains, something even the director acknowledges. And one of the traditions for that particular trope is that a fair portion of the people picked for the "Suicide Squad" are going to, um, die.
Unfortunately, probably because of sequel concerns, only one member of the Suicide Squad actually dies in the film. (Well, two if you count Enchantress, but she's the main villain and that's a whole different problem with superhero movies.)* It's a pretty decent death, done well, but it's kind of ridiculous how low the body count ends up being given the threat the Squad is facing. A few more Z-list villains getting offed might have made the threat more credible. (How do you get time for more villains? See point 3.)
*Yes, three if you count Slipknot. But who cares about Slipknot?
You have to be willing to kill your characters when the situation is so bad that nobody dying is absurd. Also...
3. Kill Your Grinning Evil Darlings
For all the hype Jared Leto's Joker got he doesn't actually do a hell of a lot in the film. His only real contribution is to abduct Harley Quinn from the Squad briefly, but she ends up right back with the damn team anyway after his chopper gets shot down! Basically the Joker moves Harley from roof level to ground level. That's it.
Joker could have been left as a flashback character to tease an appearance in a later Batman film. Hell, let him have the stinger too. The rest of the time he used up would have been much better used to flesh out the Squad, patch a few plot holes, or just introduce a couple more warm supervillain bodies to off in creative ways. Instead we got Jared Leto being vaguely creepy, and a few thousand horror stories about working with him that will dog his career for the next decade.
Don't leave a character or a scene in the story that isn't necessary. Cut cut cut!
4. Shoot The Damn Guard
You've heard that a gun placed on the mantel in Act One needs to go off by the end of Act Three, right? Well Suicide Squad loves putting guns on mantels, but it doesn't actually fire too many of them off by the end of the movie. The excepion is the neck bomb implants, which get fired off almost immediately. (Because nobody cares about Slipknot.)
A prime example is the abusive asshole Guantanamo guard early in the movie. Deadshot tells him flat out he's going to kill him. The rest of the Squad has reason to hate him. And then the Joker turns up and makes this guard his new best friend to try and spring Harley Quinn. (After killing the last guy who was his new best friend.) Yet somehow the movie forgets about this guard for the second and third act, and he remains alive by film's end. What the actual hell? Did Mindy Kaling pull some strings? (I did not recognize Ike Barinholtz from the Mindy Project but the guard is totally him.)
Do not set up a character to be obviously killed off and then fail to do it. Pick your guns off the mantel and pull the trigger.
5. Don't Let Your Heroes Let Your Villains Win
In a lot of ways Suicide Squad is a disjointed mess of a plot, but I will give the movie credit in how it treats Enchantress, the main villainess. She's being forced to work for Amanda Waller under threat of death, but manages to escape briefly by playing on her minder Rick Flagg's emotions. She can't reclaim her captive heart, but she frees her brother, who immediately begins a killing spree. Waller sends Flagg and Enchantress in to deal with it, but Enchantress bails and gets her brother to protect her from Waller. She then immediately starts working a plan to end the entire planet.
At no point in all this do the villains, er "heroes", get an obvious chance to stop her that they screw up. Even Rick Flagg only knows she's done something, not what, and it goes bad so fast that there's no time to figure it out. Enchantress pretty much executes her plan perfectly from start to finish, and it's only because she doesn't know what the Squad's capable of that she loses in the end.
I've seen a lot of plots where the heroes screw up to advance the bad guy's plot. It's refreshing when the villain actually is just competent enough to be a major threat.
Anyway. Hope some of that was helpful, and now I'm going to tuck in. Sweet dreams...