If you have a space two away from who you want to attack, what I do is drag the hero over the space I want them to go to, then continue dragging them over who I want them to attack. It's a bit tricky and you have to be careful not to lift your finger up at all.
Also the orbs are for summoning new heroes to your allies. If you click on 'summon' at the bottom of the main screens, there are a couple different summoning events you can pick from. You select which event you want, then use the orbs to summon. Each event has a list of particular heroes you can summon, so you might not get who you want if they aren't part of that list.