I have an Olympia Cremina, which is the same basic type of machine as your Elektra. Meaning it is a closed boiler system/single group. What I do to descale is first empty the boiler by turning the machine upside down to drain. Then mix 1TSP of citric acid with 38-40 oz. of really hot water and fill the boiler up. Yours may hold more water, but the boiler in my Cremina has around a 40 oz. capacity. I then let this solution sit in the boiler for several hours and then I fire the machine up until pressure builds and then I run the solution through the group, steam wand, etc. Then let it cool awhile until boiler pressure neutralizes, drain the boiler and fill with fresh water. Be sure the boiler is pretty much completely cooled down before refilling as cold water on a hot heating element may not be a good thing.
Then I turn upside down again to drain. Then fill once again, turn it on and run water through the group, steam wand until most of the water in the boiler is used up. This should be plenty of rinsing. It's always a good idea to taste a few drops of what comes out of the machine to make sure all solution is gone. Citric acid has a very sour taste.
Dezcal would be fine, but is expensive compared to standard citric acid. I bought a 2 lb. bag of pharmaceutical grade citric acid on ebay for about $15. Using 1TSP or so at a time this bag will last me a long time. Later!