To get 20 energy with Imbuing, you'd need to
1. Make an exceptional item out of regular leather with as close to minimum Energy resist as possible (3% is the theoretical lowest, but since there are 15-20 (23 on Siege Perilous) random resists added from exceptional crafting, you'd probably be looking for a 4% or 5%). POF it up to 255 durability.
2. Imbue all the properties you want on the suit, noting that the maximum you could get on a leather tunic is 18% (3% base, plus 15% from the imbue. Imbues wipe all previous bonuses above the base, such as from material or exceptional bonuses).
3. Enhance the tunic with the material required to get it to 20% or higher.
On the other hand, you could
1. Take an exceptional armor already high in Energy resist from Exceptional bonus, and reforge it with one of the properties that can add resists, and repeat until Energy resist gets added (in which case you might not even need to enhance or imbue it)
or...
Craft with barbed leather with a runic (any runic will do, the main difference is the # of properties and the minimum intensity of the properties), and try to get the resist to 20% from a combination of a runic property and exceptional bonuses - but possibly ending up with an item with too much extra properties to imbue further, or properties you don't want.
Three different options, all have their strengths and weaknesses. The latter two have the cost/benefit of being more difficult, but possibly not requiring you to have to Imbue, therefore having an item that can be repaired and POFed as needed.