I'm having trouble deciding..

I've currently leveled my Frog Immortal to level 6 and have its exclusive treasure. However, Hailey, Vet Ransell are level 6, and the Cat Mage is level 10 with its exclusive treasure, but I'm stuck on the hard difficulty. My Gold-Headed Hawk is level 6,5,5, and all others are level 3 or higher. I also haven't spent any money and have been playing for about a year, currently at level 14. But I keep encountering players who are level 9, and I can't clear it just by summoning Ninja and Blob? What should I do to beat it?? Please also recommend which Immortal to level up.