Alright, not a big problem^^ They'll come again, from hiring over time.
Skull mage still has the highest damage aid boost, 40% on skill lvl 15. So leveling his skill won't be a waste.
Blockhead is a must have on Abyss Demon. Sooner or later you might roll one, or even buy for 8k rings.
Sapphirix and Hydrasaur add damage and health, also great aids.
Another thing to keep in mind (especially as low spender/f2p). You might end up with only 1 Chiron, but a Skull mage, Hydra, Blockhead fully gloried up, that could reach level 180. This will make even lvl 120 heroes much stronger, so progress will be possible for a long time. And that's a mistake many spenders make, thinking if their rare main heroes have lvl >160 etc. they'll own the game. But because they didn't focus on leveling their aids as well, those super strong looking heroes are very weak.