They changed the decay speed on rampage so that its harder to have a fully powered rampage vs a single mob but still fairly easy to have a fully powered rampage against multiple. a warrior is stronger the more stuffs beats on him.
for that reason if i find for some reason im about dead soloing a sentry in natalan i will deliberately run out and pull a few more mobs.

to some degree you can use enduring march as it prevents rampage from decaying. does have a long cool down though but can be helpfull at times.