I read developer's comment on level 7 about arrow being blue. I think you should indicate it with upper cases. BLUE arrows are the sums of BLUE blocks.
Okay, here is how you beat the last level. Do not waste time trying to win it without maxing up your whole squad. Just play the 14th level over and over, until you have all maxed + you have enough money to buy all runes. Then, just buy all runes lvl. 3 and basically you're invincible. The health regeneration will ensure you won't die.
Here is a useful tip: In order to fast forward your progress, always focus on the next possible upgrade. Once it has 50 levels, go for the next one. This way, you can upgrade previous ones x100, without wasting time.
Pls, read LiveTactics' comment below. I wasn't able to complete final boss with all knights. But, thanks to him, I put sticky and flaming archers that helped a lot. Got it with 3 stars.