If you havn't figured it out in the last two months, then the best way to take on people in league 4 and the octopus is to use the same girls as they do. I use the Snow Queen myself (I don't remember how I got her because I have had her since the beginning); the Elder and Sire (the legendary girl and epic girl of that vampire set for water); the snowman; and the guardess. If you really go crazy and spend a lot of gold on upgrading the water girls, that will be your best bet because 1. you will be able to play keep away (that's what I call getting rid of my opponent's element gems) and charge your girls at the same time and shoot them with a concentrated amount of power. I just got into league 3 (it is very similar to league 4) and just beat the octopus completely.
If you don't have those cards in particular, try to use something in the same groups so you have the charge two times as fast and upgrade water gems to your advantage. You might also try to use earth cards that are powerful, but those can be difficult to charge if there are no green gems, and you will have to play keep away with water gems that won't charge your girls, but the newest girl set reduces the water girls' attack by 80% (although I have a suspicion that the enemy's deck adjusts automatically to negate your deck's advantages to an extent).