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If You're A Jedi Guardian/Jedi Sentinel
Calo should not be too incredibly difficult, as long as you have an energy shield active as much as possible, this is absolutely necessary, because Calo is a very powerful ranged opponent, and even if you get in close with your lightsaber, he is still a deadly threat with his swords.
Now, at the outset of the battle, you should attempt to eliminate the thugs he will have with him; these guys are weak and will not absorb much, if any damage, but it helps to have them out of the way so that you can concentrate everything on Calo.
After they are eliminated, charge in and let loose on him with all of your best skills, making sure to heal yourself often, because he will kill you if you do not pay attention to your health. If you have 2 other Jedi with you, such as Bastila and Juhani, the fight should be easy from here, you have taken away his powerful ranged combat abilities, and confined him to fighting 3 Jedi without his range, thus making him relatively easy to kill.
If You're A Jedi Consular
Playing as a light side Jedi Consular, with the right skills, this fight can be ridiculously easy. As with the strategy for a Guardian/Sentinel, you need to shield yourself and pick off his thugs before you do anything else, the thugs can be removed in 1 swift motion, such as pausing and having your whole party throw a grenade each, which leaves Calo alone and facing 3 Jedi once again. This time the fight will be much easier, because if you have high Wisdom and Charisma modifiers, Calo probably wont be able to Save Against, or resist your Force Powers. All it takes is one Force Whirlwind, and maybe a Stasis, hacking away at him all the while, and the fight is yours.