Just put luffy and a unit of each colour in your team for now and farm units to use them later

Follow this freshly written guide to know what to do

In a week or two you should start making specific teams to clear specific content;just a thing to clearify.

To use units you max each units special by feeding them copies of themselves (so you need to beat each dungeon a lot of times) to reduce their special cooldown. Then in each dungeon you use this units special to beat it or to make it easier, example : use a 2x attack special like luffy's to deal more damage to be able to beat hard dungeons

I don't know what you knew from this but this is a guide for what to do.

Now have you reduced luffy's special cooldown? If so you will beat easy content easily. So aim for fortnights