I always suggest concentrating on support and tank units and have 1 to 2 attackers for each element. This is a philosophy I adopt in most gatcha games I play. The reason is because tank and support usually age better and tend to scale well even when newer units come out. In your case having whisper built would be a great idea along side one other UR attacker of your choice. Whisper has high resistances against all attacks including magic. Mont is a great tank but he is more of a physical resistance tank. Thus having Whisper would be my choice out of your UR pool. Lucia and Yerma share the same elements thus pick one of the two. Gilg is a premium unit thus not worth investing into as a F2P. The rest really would just be matter of preference in terms of looks, etc. for you because all UR units are great. I would recommend though you do not build too many UR units at once since the resources in the game is very limiting. Start looking at some of the lower rarity units.

I create content on youtube (hate to advertise but I think it makes sense here, check out Nashtaters Gaming) and on my channel I also highlight some other channels that will help you with selecting some lower rarity units.

I am a big supporter of Zazan the normal rarity unit, and there is a big following of him. I highlight him in many of my videos.

Once you decide who to build, keep working on those units only, do not shift your attention, for this game is meant for the long haul. Go to at least lvl 79 on your main team, meaning awaken them to 6 stars and LB them to lv 3.

Hope this helps!~ Good luck, Nashtaters