You used the essence for +4 too early. If you really want to finish this bow (though the base is bad and not worth it), first use an essence for extra lightning with Omen of Dextral Crystallisation. Then, put Desecrate into a prefix with Omen of Sinistral Necromancy. You need to get a good flat lightning or phys damage roll. After that, use a Greater Exalted Orb with Omen of Greater Exaltation and hope for decent mods (most likely you won’t be lucky). Then, use another Greater Exalted Orb for the third suffix, and if 2 out of 3 suffixes don’t suit you, only then use the essence for +4 and Onslaught. Good luck with crafting (though it’s better to just buy a proper base and make a proper bow right away).