When you have a Harpoon and are trying to kill someone else with it, shoot the ground around their feet. This is much easier then trying to shoot the person's body. You may even kill more than one. Note: Do not try this with other weapons such as pistols, machine guns, commandos, etc.

When your opponent uses his or her shield, hit them with your stun power. They will be unable to hack out of it because their shield drains all of their power. Then, pick them up and throw them. When your opponent uses MRI Vision, use your shield. You will become invisible to them. Try different combinations of weapons to see which works best for your fighting style.
Complete All Stages on HARD difficulty

Right, Left, Right, Left, Down, Down, Up, Up
L1, L1, R2, R2, R1, R2, L1, L2
Down, Right, Down, Right, Up, Down, Up, Left
Down, Left, Up, Left, Right, Down, Left, Up
Left, Down, Up, Right, Up, Right, Left, Down