I would recomend renting a stump grinder if the bed size does not necessitate a trackhoe. The cost is normally half.

You have 3 options:

1. Burn them out - slice them close to the ground and bore holes in them. Saturate the holes with diesel. You can also try mounding a bag of charcoal on top of the stump and lite it. A large stump can burn for days. This method isn't always practical.

2. Go to Home Crapo and rent a stump grinder. Half a day runs about $75 or so. You can usually grind them down about 8-12". This method is still a lot of work.

3. Dig them out. Either by hand, backhoe or bulldozer.

peterc38, plantman314, and davefr, thank you for good suggestioins!
I decided to leave huge stump and just get rid of large roots under the ground in my flower bed.
It is tough job and need to be patient but only need to do once and I get more than enough exercise.

davefr left out one option:

4. Let it rot :)

If you are a glutton for punishment and have plenty of time to spare, by all means do it by hand.

The HD grinder is an option, but if you have access and enough stumps to grind, I'd recommend renting a larger grinder from a tool rental place. The unit will probably be self propelled and require a trailer to get it to your location but it will chew up the stumps in a fraction of the time the HD large lawn mower sized grinder will do it. Plus the HD grinder will do 1 maybe 2 stumps (depending on size) before the teeth dull.

depending on how big the roots are and how thickly they grow together, you MAY be able to use a tiller to rip them apart.

another option is to simply built the bed UP. put several inches of new topsoil and amendments on the ground and plant there. leave the roots alone.