I dunno.. maybe it's just the heroes you are using? I think I lose 1 GW every 10 days.. or less.. There are some tactics that work very well.
the only battles you should have a problem with are 6,9,11 and 12.
The others you should be able to beat with whatever team you have in your pvp/arena.
Remembering that sometimes Kill order matters (which one you take out first)
Sometimes your leader matters (switch it up)
sometimes I send in a suicide squad (as little as 1 low level to all 5 low level heroes) this gets the CPU to use up their special attacks.. yeah.. I hate Leia invisible, or trooper solo taunt.. I send in 1 toon, and its out of the way.
Many people think they need to use all their specials throughout the entire fight, I only use them at the BEGINNING of the fight.. Make sure all your people are green health and all the cooldowns are done and ready for the next battle (as many as possible). when there is one or two of the enemy i use only basic attacks unless i need to throw a heal in .. keeps your people ready for the next battle.
when your team is vastly outmatched.. you may need to send in a middle range team with the sole goal of knocking out "protection' .. or just one hero on the other side (Maybe their tank?)
Seriously though.. I think I have a better than 90% success rate..
if I lose someone prematurely.. you can always just reset and swap around a few things