Similar to most games of this type, you can't really control your troops, other than the selection and special attacks. Therefore as long as you upgrade all troops evenly and have a generally equal amount of each type in battle, you shouldn't need to grind... at least until the last 3 levels where I had to grind my way to max levels on everything to JUST barely beat the 2nd last island with 40 points left. Then I fight the BIG SCARY BOSS and he's the easiest enemy in the game, beat him with 4000 points left over and watch the REALLY lame ending. Disappointing...
Anyways, the rescued heroes could have been better spaced out since the lack of new strategy makes the middle levels slightly boring. I agree that the 'level estimates' were ridiculously off. Even without grinding I was on level 32 before I ever reached the 11~24 island and I beat the game on level 39 (not 50). Maybe just have them numbered 1~25 instead.
This game included many elements form other game styles such as runners, upgraders, empire builders, but that actually dilutes the strategy as well since there is no specialization. Ultimately it is fun to play and beatable for anyone who puts the time in. The graphics are great and it is a well made game overall.