I realized I have some dead cards in my Vial Smasher and Thrasios EDH deck and have 4 nonland slots to fill. I can't decide which 4 of the following to use. What do you think would be best?
-- Lightning Greaves
-- Eternal Witness
-- No Mercy
-- Snapcaster Mage
-- Sylvan Library
-- Training Grounds
Decks are about a matrix of synergy, and unfortunately we have no context to your 99 unless you post a list we can see.
This. And with that said, both Training Grounds and Sylvan Library have great synergy with Thrasios and were auto includes in my list.
Going a step further on the efficiency ladder i'd say Mirri's Guile is an option too.