Pick up a Nidorino/Nidorina from the Byxbysion tunnels. In Nidoking, you've got great offense, and in Nidoqueen, great defense. The only issue would be finding/mining moon stones.
There's always Garbodor, but it's not got much going for it beside Toxic Spikes.
Muk can be good for attack, but its moveset is poor.
Amonguss would be excellent for eating up physical attacks, which your team doesn't seem to have a remedy for, but you already have a Grass/Poison.
Roserade is an excellent special attacker, but again, you've got Grass/Poison. Also, Venemoth fills its role just as well.
Drapion is a great buffer against Psychic types and would make a good physical attacker to add. That's all that comes to mind, currently.