First off, to level Synergy, worry less about actually completing crafts. You can level up to the synergy's cap with a single set of ingredients simply by failing over and over. You have a chance at getting a skillup on any action you make. I actually recommend Thwacking despite what people say; sometimes when you're rushing due to time you may push over your threshold, and that one thwack has saved a synergy for me a few times. Thwack, Feeding, and Safety Lever are the three you should focus on, in my opinion.

But yeah, find a recipe that's no more than 10 levels above you, and just spam one fewell type that's relevant to the recipe. Try and thwack explosions for skillup chances, and otherwise just keep pumping fewell. It's how I capped off.