Get as much buckets as you can. Fill one with water and the rest with lava. Hope you have a big lava source for this one. Now, how many lava buckets do you have? Makes as much compartments as there are lava buckets, like so... (3 lava buckets)
OOOOOOOOOO
O O O O
OOOOOOOOOO
Pour one lava bucket into each hole. Take out your water bucket, pour, collect, pour, collect... etc. Then you made yourself __ pieces of Obsidian. Only a diamond pick can mine this, so go look for diamond.