I know this is a bit old and already have answers but I'd like to include my own answer
I looked on Marriland and saw that three of your Pokemon are weak to Fire, two are weak to Rock, two are weak to Bug, two are weak to Flying and two are weak to Ice. The best dual-type combination that can handle those weaknesses is Water-Rock.
The only Water-Rock type Pokemon you can find in the Alolan Pokedex is Tirtouga/Carracosta.
Therefore, I will include a suggested moveset for Turtouga/Carracosta
Carracosta @ Mystic Water / Hard Rock (boosts the power of your STAB)
Carracosta has a solid Attack stat and a great Defense stat making it a bulky physical attacker. It's slow but for in-game, it doesn't really need to rely on speed, just attack and win.
Or if you don't want to use Carracosta, your next best bet would be a type combination of Water-Fighting.
A good Water-fighting Pokemon would be Poliwrath
Poliwrath @ Mystic Water / Black Belt (boosts the power of your STAB)
Ability: Water Absorb
Submission / Brick Break - STAB
Body Slam - strong Normal-type move, has chance of paralysis
Bulldoze - covers Electric-type weakness
Waterfall - STAB
Poliwrath has solid defenses of 90/95/90. It's Attack stat is also good and is a bit slow but again, not really important for in-game use.
Hope I helped!