I use a spray on oven-cleaner and follow the directions on the can exactly. This involves heating the oven to 50 degrees centigrade, turning it off, spraying the cleaner on to the sides, top and bottom of the oven, and leaving it til the following day.
Then wash off the cleaner, wearing rubber gloves and using lukewarm water without any detergent or other cleaning agents, and a cloth you can throw out afterwards.
As you don't clean your oven often, you will probably have to repeat the process two or three times.
No elbow grease involved.
As I buy my oven cleaner from the German firm of Rossmann, there is no point in mentioning its name.
In future make life a little easier by cleaning your oven more often. My domestic science teacher startled us by addressing us in these memorable words, " Ladies, clean the oven every time you have used it, using a brush and hot water, then wipe it using a soft cloth."
None of us dared ask her if she did this after serving Christmas dinner, I may add!
I had a vision of her in her Sunday best, apron and rubber gloves cleaning the oven before allowing anyone to have their presents!
I only clean my oven every three months or so, and it is used a lot, but always wipe it down after roasting meat, or cooking anything else that tends to splash or spill.