If you've decided you want a flower bed right where that huge rock sits in your back yard, don't waste your time trying to move it. Instead, use the rock as a strategic part of your flower garden. Large rocks can serve several purposes in a flower garden, and you don't have to tell anyone you didn't plan it.
Use large rocks in lieu of benches or garden seats. If the surface of a rock is somewhat level and flat, it makes a great alternative to a garden bench or wrought-iron chair. Plan a small footpath through the flower garden, leading to the rock, and you'll find yourself ---and visitors---naturally wandering down the path and enjoying a moment to rest and enjoy the view right there in the garden.
Use large rocks in the border of a flower garden. You can allow a large rock to mark the line between the beginning and the end of a flower garden; its looming presence there at the edge will signal a transition, and no other edging may be necessary. Or you can create a low stone wall around the flower garden with the large rock as a centerpiece.
Use large rocks to protect tender plants from too much wind or exposure to cold. Plant tender little flowers on the south side of a large rock for a natural windbreak. Plant taller, heartier flowers on the north side so they'll have something to lean on for support.
Use large rocks as a focal point in the flower garden. Plant your loveliest flower selections all around a large rock, and allow the rock's sturdiness to create a dramatic contrast with the delicate petals and soft shapes of the flowers.