Getting a cotoneaster out

I am grubbing out a cotoneaster, I think it's a franchetti. Although it's fairly small because I have hacked it ruthlessly for years it has a good big trunk about 5/6" across. I've been digging down around this, cutting off any roots I can find but it doesn't really want to come out of the ground and I just don't think I've got the strength and energy to haul it out. If I scrape away the soil down to about 6" would it be OK to just saw it off about there? Would it regrow?

I could treat it with stump killer, except I will be wanting to grow tomatoes on that spot as it will be part of the new greenhouse. How will it affect future plants and crops?