My strategy with my team, if I am playing with them, is to try to hold that top Laundromat Spawn. I typically cover the blue building, watching the breach in the wall and the doorway. Others cover the Graveyard and Mechanic Shop.
I've got more of a comfort zone now on that map, however, so I move around a lot more than I did when this game first came out. What works for m me is to work the perimeter. If I spawn parking lot, I either
1) Flank through the Blue Building (This can be tough as the entrances can be covered from range, but if you enter and immediately hug the right wall they have trouble seeing you. Make it out the left window, go around the Humvee and pick people off from the pallette there.... then move up gradually until teammates spawn in that spawn with you),
2) Go up the right staircase then hook towards the fountain. Always check the middle for campers on the fountain, then go right to the box on the corner and peek over. If all is clear I sprint for the mechanic shop then go right around the back alley way and flank their spawn, using the smaller fountain as cover.
3)Same as #2, but go into the Graveyard instead. Peek over the barrels to clear out anyone camping there, go in strafing right while looking to my left to see if anyone is covering that entrance from the vacant lot. When it is clear I go through the right arch and hug the wall next to red van watching their spawn until I am able to flip it.
Its been rare lately for me to have bad games on Slums. I think what has worked for me is knowing where people tend to camp and using Grenades and strafing to my advantage.