Collectors hit hard. Very hard. Wall of text incoming....
Some general things to help you with collectors:
1. Always check your six. Praetorians are sneaky and given their bugged sync-kill (like phantoms and atlases) and huge size, it can be difficult to escape. Seeker plagues and swarms are pretty dangerous too and often hit you from behind.
2. Focus on abombs. All but waves 6 and 8 of collectors have abombs, and focus on killing them. They are worth 40 points to the wave budget (which is a lot for such an enemy). To put things in perspective, killing 3 abombs is equivalent to killing 1 praetorian (120 pts) as far as wave depletion is concerned. Check the resource library for budget points for all enemies.
3. When a praetorian fires a lazer or launches missiles, take hard cover. Soft cover doesn't always work. Hard cover works VERY well (you might take some damage if you're too close to the edge, but it's better than standing out in the open and dying, and then complaining). Or you can move around and trick the crab into lazering else where. Same thing for scions, if the first blast hits you in soft cover, move or dodge to avoid the other rounds hitting you.
4. The best faction for headshots. Praetorians have a 2x multiplier to headshots while all bosses have only 1.4x and praetorians are FAR easier to headshot than the other three. Even the troopers and captains have gigantic heads, making for easy headshots. Shoot scions in the sacs for spike damage. Praetorians are also the easiest boss to stagger, and constantly headshotting them will cause them to tumble giving much needed space and time.
5. Keep on the move. Collectors are excellent at trapping you with grenades, lazers, seekers, etc. Don't stay in the open for too long.
6. A trick to deal with possessed scions - if you are in soft cover, it'll lob grenades, flush you out and pummel you with it's cannon. Since it alternates between grenades and cannon, stand in the open next to a soft cover when it's about to throw nades (now, the nades are in the open and the cover is free!). Then go back into cover and use RHA. Rinse and repeat. Works well most of the time (susceptible to lag).
7. For isolated scions and poss. scions, go close and trigger its melee attack and back away immediately and start shooting. This way, the scion is a big puppet as it doesn't shoot or lob grenades during that time. Dance around the scion and make it look like a dummy (doesn't always work off host)
8. Praetorians have a cap unlike other bosses. Only 3 can spawn on wave 3, only 6 on waves 6 and 8. (not sure about w10 and w11). Check in the resource library for these.
9. You can trick seeker swarms into blowing up. Go close and when you see it vibrate, dodge or move back to avoid the explosion. You can do it with plagues too but you need to time it well and you need to have a dodge (preferably one that covers long distances in short time).
10. If you're on PS3, pray!