Engineer

Engineers are augmented humans who are rather difficult to eliminate using conventional weaponry. They are only vulnerable to energy, acid and frag damage, as well as PSI strikes. While the engineer doesn't shoot or move up close, he is constantly throwing grenades at you. You can keep catching his grenades and shooting at him as he doesn't have a shield to cover behind. This is a small advantage though as he periodically summons two turrets to keep you busy.

Weapons inflicting acid damage - for example the Acid Pistol - work best against him. It is also advised to have a squad member with an acid damage weapon - such as Rico - and plenty of grenades (acid or frag).