Beware of the glitch in the trophy which is annoying as all hell. I managed to get one Sentinel to level 30 with no trophy.
The trick is to make sure you get to level 30 IN the actual mission you completing i.e. do not get the level 30 with bonus XP AFTER the mission as then the trophy will not unlock and you will need to start again.
At least getting the sentinels to level 30 doesn't take too long if you have not managed it already.
Also trying adding mods every time you level up as that makes the sentinel more powerful which leads to more XP being gained due to kills, etc.