Several seconds is over exaggeration, I suspect it's less than 1.5 seconds. The Watchers should also be the #1 enemy to take out during an encounter because of their ability to back up the other Prometheans. During my Legendary Solo run, I literally think I allowed a Watcher to revive an enemy ONE TIME, and I was pissed I let it happen.
4-5 weapons? Wha? I'm using the LR as the example below as you have access to that weapon a a good majority of the game while fighting Promethean enemies.
LR can hold max ammo of 120 bullets, or 40 trigger pulls while scoped in.
IIRC, Watchers take 4 (5?) trigger pulls while scoped-in with the LR to take down. That's 12-15 bullets each, so 24-30 total.
120 - 30 = 90 bullets remaining to take out the 2 Knights. Knights on Legendary take an entire clip to drain their shield, so that's 36 rounds + 2 more trigger pulls to remove the helmet then a headshot, bringing the total to 42 bullets. 90 - 42 = 48 bullets remaining, allowing you some wiggle room to take down the last Knight.
This of course assumes you hit with just about every single trigger pull with the LR, which more than likely doesn't happen. But an encounter with 2 Watchers and 2 Knights should certainly not drain away 4-5 weapons, unless of course you're carrying garbage weapons. Carrying anything other than some type of Rifle and a Plasma Pistol, or a Power Weapon (Sniper/Rockets/Shotgun/Needler) is your own doing.
All that being said, the campaign on Legendary Solo can definitely be goddamn tedious at times. But, having fought Flood for 3 games, I more than welcome the new Promethean Enemies.