To unlock the Sentinel-class in Tribes of Midgard, you will need to block a total of 25 attacks in 10 seconds, a very tricky task but not impossible.
In short, you will need to block and hold the shield of any class while taking damage, and you will need to take enough damage to number 25 all in 10 seconds.
So what you could do to complete this task is something similar to what we recommended for the berserker - for this method you will need to build a gate.
That said, once the gate is built, wait during a horde night, and let the helthings pile up on a gate and then open it and stand in front holding the shield and blocking.
If luck favors your boldness, you should get attacked and complete the challenge to unlock the Sentinel-class in Tribes of Midgard.
However, it's worth noting that this method might not work as the helthings will probably run past you towards the tree. But don't you worry, as that is not the only trick we have for you.
The next and probably best way to unlock Sentinel is to go to the Land of Pools and cart a ton of goblins and hold block. Goblins attack really fast, and in a large group, you will hit 25 attacks in 10 seconds with no problem at all.
Check out some more Tribes of Midgard guides at Pro Game Guides!