The door and lock are the best engineered and strongest part of the safe, so stay away from those. Use an angle-grinder and cutting wheel to cut the back or bottom off the safe.
It depends on the safe. The ones I work with have to be open to reset the combo. I would recommend contacting the manufacturer. You might need Proof of Purchase.
Even without the combination there are different ways to open a safe. The easisiest way to open a safe is to use a magnet. You put in near the locker and you can open it by moving the magnet in different directions.
Well, you can get a lever and try to pull it open. Some safes come with alarms, so it may be difficult to do this. Unless you are opening your own safe, I wouldn't recommend such a step.
For most safes the only other way is to get a professional locksmith to drill out the lock. This can cost quite a bit because you will need to replace the lock. Try to remember the combination next time!!
It the safe is only of a flimsy construction a decent crow bar should do the trick. Otherwise without special knowledge you would have to try every combination