How to be an effective sentinel in Mass Effect 2
Introduction Sentinels in Mass Effect 2 are a hybrid class which combine tech and biotic attacks. They use the tech powers overload (which damages shields and synthetic enemies) and cryoblast (which freezes unshielded enemies) and the biotic powers of throw and warp (which does extra damage to armor and barriers). But they also have a unique move called 'Tech Armor' which provides a layer of extra shields for the sentinel and fires a destructive shockwave at enemies when destroyed. This is the bread and butter of many good sentinel builds, and the build I am proposing here is the 'power sentinel' build.
Powers to Use This build focusses on putting all for ranks into tech armor and choosing the 'power armor' upgrade over the 'assault armor' upgrade. Power armor adds even more shielding to the tech armor and adds a 15% power increase to any tech or biotic attacks you use while the armor is up. The sentinel's passive ability when maxed gives even more of a power bonus to your tech/biotic attacks.
This amplification means that a sentinel with a high passive ability and power armor can make use of extremely powerful tech and biotic attacks. The bonuses from the passive skill and power armor will improve the warp skill to further harm barriers and armor and improve overload (or optionally the bonus power Energy Drain) to further damage shields and synthetic enemies. Once an enemy has lost its shields/barriers/armor, throw or cryoblast can be used to incapacitate that enemy. This means that your sentinel has an answer to any enemy be they shielded, armoured, covered in a biotic barrier, or defenseless.
My recommendation would be to put points into throw, warp, tech armor (and choose the power armor upgrade), defender (the sentinel's passive skill), and the bonus skill energy drain. The reason I'd recommend energy drain over overload is due to its synergy with tech armor- energy drain will damage an enemy's shields and increase a percentage of your own, which is a great way of recharging the ridiculously high amount of shields that you will have due to your tech armor.
Strategy This particular build will focus on being at around mid range on the battlefield. The reason for this is that being at long range means enemies are out of range of your powers and being at close range will mean that you'll constantly have enemies destroying your power armor (which you want to maintain for that nice +15% power bonus). This means that medium range guns are great- the locust and tempest smgs are good (the tempest is better at medium-close range while the locust is better at long-medium range), the handcannon pistol is also good. When you come to pick your third gun type, you'll want to pick assault rifles over shotguns and sniper rifles as they are more effective at the range you'll be dealing with.
As well as firing your guns, you should also focus on spamming your powers as much as possible to cause as much damage as possible. Remember to use the right powers on the right enemies!
As squadmates, you'll probably want high defense, close range characters such as Grunt and Jacob to go in close and keep the enemies pinned down while you hit them from mid-range with your powers and guns. Be sure to order them onto the frontline to tank for you.
Conclusion So, with your power armor up, your assault rifle in hand and your arsenal of biotic and tech powers, stand strong at the mid range of the battlefield and support your squadmates who are attacking the enemy at close range. This is an extremely tactical and rewarding form of gameplay which demands that you use the right abilities at the right times and make effective use of your squadmates to win battles. It is very versatile in that you can play different roles on the battlefield if necessary- although I recommended staying at mid-range, it is also viable to get in a little closer (due to the huge shielding that tech armor provides!) or even go a little further out of enemy range (as long as they are still in range of your powers).
Hope my guide helped, have fun.