Of what's currently available, wind is the easiest for a free no cash player. You can hit -50% atk/def using Hraesvelgr, Joan+Ambush (unlock with soul p), Cybele (from story) and Zephyrus (from gem gacha) with 2 free hime slots for whomever.

Fire is somewhat easy too with various debuffers in gem gacha for starting out but lacking events right now so may have difficulty strengthening.

Thunder is the one I don't recommend, especially if you're not going to get any miracle tickets. Water is kind of in the middle but leaning towards the same issue as thunder.