Wowow why do you make it so difficult for yourself? He's Grass and Water, so he'll take 1x from Fire... why are you doing all this crazy math?
In regards to your first post, yes, easiest way is to familiarize yourself with single typing weaknesses and put them to work when you see a dual type pokemon. With time you'll be good and know every pokemon's weakness/stregnth. Don't forget nulling abilities such as Levitate, Dry Skin, Volt Absorb, Wonder Guard, Water Absorb, Storm Drain, Sap Sipper; the list goes on. E.G:
Quagsire is Water/Ground, so normally it would take 1x damage from Water moves, but with the Water Absorb ability it heals 1/4 of its HP when hit by Water, so you have to find another weakness (aka Grass and Freeze-Dry which is not ingame as it's a gen 6 move).