Here are steps to claim space from Mint partition.
Login to Ubuntu. Start gparted
sudo gparted if not available then install gparted.
Find a Mint partition, Use format to OR delete and create new partition.
If Mint have modified (taken control of) ur boot loader then follow next instructions:
Boot from live USB/CD for Ubuntu (I use same version I have no idea how different version of Ubuntu would work specially 32/64 bit differentiation).
If you don't know the drive where Ubuntu is installed use
sudo fdisk -l it will list partition. Note down one with root and Boot will be indicated by '*'. Assuming your root (/) partition shows /dev/sdaX.
sudo mount /dev/sdaX /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo chroot /mnt
grub-install /dev/sda (NO X here) Assuming you have only one HDD and that is sda
8. Once it is installed unmount all
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt
Reboot (Remove USB so you can boot from HDD).