I want the explosive minigun but im finding it dificult to locate enemies
anyone know any good grinding spots

There are a good few set encounters with legendary enemies, a quick google search should get you some results. For random legendary enemies just ramp the difficulty up to survival for maximum chance of one spawning. There is also a save/load thing you can do once you find one. Save before fight, reload if the drop is crap. Haven't done it myself so there might be a little bit more to it than that. Luck might also have a factor in what drops, some people seem to get epic drops, others just get rolling pins and pipe wrenches.

As i understand the type of weapon is set on cell load, but the legendary effect is set on death so saving then you run into an legendary should give you the option to reload and get another effect if you find a nice weapon with an weak effect and try to get an good one

As to specific locations, I have good luck here:

When you leave the stadium @ Diamond City, just down the street where you often hear all the gunfire, there's almost always security fighting SuperMutants in a ruined two-story apt. building. Seems like there's almost always a legendary among them.

What difficulty are you playing on? On survival mode it seems like the majority of enemy encounters have legendaries, and the fights are more numerous. The weapon drops are still mostly useless, but it's a numbers game.

The National Guard Training Facility is probably the easiest and most reliable, although I've heard some people complain that they don't get a very good variety. I only had to do it a few times myself until I had a pistol (bleed damage) and sniper rifle (violent) that I decided to keep.

First, right outside the door, turn it up to Survival difficulty. It all but guarantees a legendary ghoul will be inside. Quicksave in front of the door. When you enter, the game will autosave. Do not open your Pip Boy or do anything else that causes another autosave at this point. There will be four ghouls inside. Kill them, and check the legendary's body. If you like the item, but not the effect, reload the autosave. The item will remain but the effect will be different. If you don't like the item, reload the quicksave.

Once you have something you want to keep, you'll need to wait 20 in-game days for the place to reset to do it again. (Or find another location, many recommendations can be found via google.)

Dunwich boers, hub city auto wreckers, gunner plaza, mass pike interchange, glowing sea, quincy

Hubris Comics is the fastest and you have 2 Legendary Enemies near each other. and not far away from the entrace. Only Problem seems to be that they dont' drop Gatling Laser, Magnum and Stormriffle. Gladly i got The Explosive Minigun and the Explosive Combat Shotgun there. ^^

Use Quicksave before entering Hubris Comics for the Item Roll and Autosave for the Affix Roll.