To improve you need a decent attack. Focus on defence once you have that. For what you have at the moment I would suggest you ascend Green Shiva and make sure both G Shiva and Negans Active skills are at the point where they can go off on turn 2. The reasoning behind this is that at your level I am assuming you are struggling with Shields that have stun/impair/abs def/Ap down.
With both Shiva and Negans AS’s mixed, you can build your ARs up on a different toon turn 1, then on turn 2 the shield will likely have a stun or taunt resist but is very unlikely to have both. Use the skill that they can’t resist, then if need be use the other skill to control another toon like Mercer. Then carry on building up your ARs on toons without those troublesome defensive weapons. You will only then have to go through 1 hit each that could be affected by impair/stun etc.
The other 3 toons I would take would be Camila as lead, James and Lacerator Shiva. James gives you a huge leg up to catch up with others in your bracket as he balances the playing fields somewhat even though your opponents will also have him. Make sure all 3 of these have their ARs levelled up first with Lilliths/Adens and G Shiva and Negan taking up your Active Skill trainers, until you can max them all.
Y Shiva and James work well together and their combined bleed damage will synchronise nicely with Camilas Burn damage. These 3 will be the workhorses of the team.
Make sure all of your toons have at least a very large Ap on attack weapon. This is KEY. So build up your weapon factories and research V large Ap on attack for green and yellow asap. Also worth researching attack boosts to add to those weapons.
For mods, if you have much choice at your point, you want to focus on attack sets. Camila needs your best attack set stun resist as she can then cleanse others of stun, but if you have multiple, stun resist is a good one to have on as many as possible, if not, then impair resist. Ap down mods for bottom left are always one of my favourite but bleed and burn will work for this team as well. Reflect and Graze are also good ones. For top right either go attack against your advantage or defence against your weakness.
The big weakness of the team above is that it has no healers or revivers. I would have suggested a Carl lead but it wouldn’t be worth it to leave out James. Instead of G Shiva it might be worth including Carl in your team even though he won’t benefit from the Lead, just so you have a healer and reviver. If so, make sure his mods and weapons aid in defence over everything else as you don’t want him to go down (but still with an Ap on attack slot).
Oh and stop spending money on speed finishing your buildings. When you have them all maxed you’ll be kicking yourself for spending it on them.
Edit: I’ve advised to ascend G Shiva but she would be the first I’d be looking to replace with future toons. So if you are strapped for resources, just put Carl in straight away.