On the left side you have this nifty overview window that shows all your planets (called empire tree). It usually jumps to the planet you are fighing on, so you can use it for micromanagement. If you minimized the overview of that planet you need to maximize it again, also select "group frigates" in the upper leftern corner (othewise it's too clogged up in large battles to be used effectively) and select those ships that you want to use for attacking the enemy hoshikos. Then left-click on the hoshiko symbol that you can see in the empire tree, not on the battlefield. The fleet will now focus-fire on the Hoshiko.
Unfortunately they will only destroy one ship and search for the closest available target, no matter the ship-type again. So you will have to keep an eye on it and reassign them the target. But you won't have to search for every single Hoshiko in that cluster of ships anymore, a huge advantage already.
In your case you might want to go a bit further, select the bombers in your gravity well from the empire tree (left click on one of them while holding down ALT) and focus them on the Hoshikos. The bombers usually auto-attack various targets one by one when they could easily overwhelm even enemy caps by focus-firing. They can be even more effective than your mainfleet if microed enough.
You can also use the overview for other nifty things, you can assign planets o send their freshly produced frigates to another planet or directly into one of your fleets, you can scuttle useless vehicles or buildings without having to move to the gravity well they reside in etc...