There’s two spells specifically that work on shaman that can generate skulls, Yellow/Scorpion Juju, Green/Spirit Mask.
For Merc, best green spell is Dirty Blade for skulls, but you need Greed active (which is way easier to get now thanks to Buccaneer pants).
If you still are lacking, you can also get the Esgaard Bandolier and 2of6 Runic, which can give 1 and upwards of 5 skulls for every spell cast, depending on how many red gems are still on the board. I use the Runic boots and ring for speed and ult mana.
With that setup, I usually have more skulls than green by the time I get through all the spells.
If you happen to use a Green Alt Assassin, I use Darkfire Gaze as my last spell, and I started using Cleansing Doom (the season 2.5 Green Spell), swapped in for Dispel, and it’s good for green and purple spells.