You actually have a lot of good units. Fire vamp, Verdehile, and fire epikion priest, chloe, are some of the most coveted monsters in the game. You also have fire dk, Arnold which is really strong in pvp and dungeons, water Sylphid, which is good for dragons, Jean, who has aoe defense break, and raoq who is a really strong farmer. The units I stated above are the ones you should work on. All of the above monsters have high late game potential and don't require amazing runes (except verdehile, who needs 100% crit rate). In your position, I'd work on Arnold first as he is useful in all areas of the game. Keep Verde as a side project until you know you can get good enough runes on him to have 90+ crit rate. You also have Raoq already, but he is at 4 star, so I'd 5 star and rune him ASAP as he is a really solid attacker and farmer. In order to begin farming giants effectively you're going to need to find shannon, which is wind pixie. She has a really strong 3rd ability which allows your monsters to be artificially tankier than they really are. Bernard is also a very solid giants monster due to his 2nd and 3rd abilities, and I'd recommend leveling him up as well. Some other really strong monsters that you'll need to keep an eye out for are the light trio: light bearman, light inugami, and light vagabond. Once you've found these monsters, you'll be able to progress through giants very easily. As for the scenario missions, if you haven't yet cleared them, go ahead and use friends' monsters. However, the rewards for finishing everything after faimon are disappointing, just one mystical scroll.

Sorry about the disorganization, wish you good luck in your endeavors!