Okay, I'm going to try and take this guy's rather vague suggestion and try to flesh it out.
Sentinel's purpose is to act as a form of tank for the team or as a wall to physically block and deter ZEDs in enclosed corridors. To accomplish this he uses large riot shields and various other barriers that either stop ZEDs from pushing forward or block attacks for a certain amount of damage. In order to level the perk you must either block a certain amount of damage with your shields or deal a certain amount of damage while wielding a shield.
For discussion pusposes lets think of things that the Sentinel can actually do.
1. Wields shields. Shields come in a couple different types the basic Riot shield is great for stopping melee attacks and Siren screams but doesn't work against Husk fireballs or Patriarch Chaingun. However, it has a lot of HP. The Blast or Ballistic shield can deflect fireballs and Patriarch chaingun and can reduce the damage of Patriarch rockets, but it has less HP than the Riot shield. Shields are extremely heavy and make everyone move slower, but the Sentinel will get a bonus that mitigates this downside and makes him much better at using them.
2. Can wield small arms and melee weapons while holding out a shield. This is basically where a Sentinel comes in as a combat perk. When not blocking, he can fire handguns and other 1-handed guns. He can also use melees, but can only use light melees like Machete and Katana because he only has 1 hand available. He cannot use iron sights on any guns because that brings up the shield.
3. Can use shields to push ZEDs or stun them. This will probably be a secondary attack, causing some damage. A shield bash has some knockback that can shove light to medium ZEDs back and can disorient them by hitting them in the face. Fleshpounds will usually just get pissed off by this, though it does distract them for a moment.
Now, perk bonuses. Of course he has to get bonuses for shields and get effects that make him better at holding back the swarm.
+x% shield HP.
Longer bash knockback.
Longer bash stun times.
Faster movement speed while using a shield.
Better armor.
Maybe increased max HP?
Other advantages would have to be discussed, but that seems like a pretty good basic roster of perk bonuses.