To increase the efficiency of a portable locate the condenser intake and build an air box and add an intake pipe outside. That way it's not sucking air that it worked hard to cool, and putting it back outside.
Something like this:
Also, get the biggest you can get. That one is a 1 ton unit. It helps cool down my office and the hallway, and if I point it at the return vent and run the furnace fan it helps somewhat to cool the house but not quite enough to do the whole house.
Keep in mind they are very loud, and also check whether or not you need to empty water. Some of them require you to move the entire unit over to the bath tub, lift it inside and let it drain. Make sure you don't get one of those!
Mine actually evaporates the air through the exhaust so I never have to empty any water.