You can use a dry yeast in a starter. The idea is to multiply the yeast, dry yeast multiply just as much as wet yeast when added to sugar. If its a big 5 gallon batch you'll want a nice big starter, I'd do a 2 liter one for most brews. You'll just want to check the stats on your yeasties and make sure they can survive the alcohol content.
Edit: A half cup of slurry is a bunch of yeast, probably equivalent to like a 1L starter, imo close enough for that gravity. I do starters for ever beer above 1.050. 05 is a BEAST though, swear if you put your hand in during fermentation it'll eat it too.
The best beer I ever made was the next one I brew.