In my native language, we can use future continous to say a thing we are going to do in a short time or right at that time. For example, my boss assigns me a task and I would answer (literally translate) "I will be working on it right now".
For more context, it is used when I am about 90% sure that I start right at that time. But I may also finish whatever I am doing at that time, then start the assigned work later (but still in short time). That sentence is used to ensure my boss that: ok, I can do that, and I am going to do that and finish quickly.
Is it valid in English or is it weird (in either writing/speaking and formal/informal context)? What is the best and most common sentence/tense to say at that time?