The Krogan Sentinel and Solider are considered "rare" items for a reason. Their unique Krogan abilities are a charging melee and rage.
Rage is an ability that triggers when the krogan performs 2 successful
melee kills within 30 seconds. this grants the krogan increased
protection for 30 seconds. this effect can be renewed for each
additional melee kill within the next 30 seconds.
Combining this with Tech Armor (Sentinel) or Fortification (Solider) allows the Krogan to become a bullet sponge tank. The power cool down penalty has less of an impact on a melee heavy game play strategy. That being said focus on weapons over powers. Finally, the Tech Armor explosion helps minimize the melee vulnerability.