As @Nullings said, if you have enough gems/$$, you can just keep running fast teams, which don’t take long at all to run and keep racking up points.
To address your other question:
TLDR: Rushing to 500 using shortest path and then clearing the floor repeatedly with fast teams gives you more points in less battles than full-clearing all floors up to 500. It does cost a lot more gems for the same amount of points, but if you want points/leaderboard, you’re probably prepared to dump gems into the event.
Calculations and explanations
During faction assaults, each room has an extra reward (don’t have a screenshot), which states the number of points. Like all the other room rewards, the multiplier affects how much you get. At level 500, each room gives you 2500 points, and the boss room give you 10000, which gets multiplied by whatever multiplier you have when you defeat each room. At level 20, this amount is 100 and 400, respectively (i.e. level x 5 for room and level x 20 for boss).
1st room (always 1.1x) so 2.1 x 2500 = 5250
2nd (say 1.3x) so 2.4 x 2500 = 6000
3rd (say 1.2x) so 2.6 x 2500 = 6500
Boss (always 1.25x?) 2.85 x 10000 = 28500
Total for run = 46250
If you’re using a fast team, it shouldn’t take long to finish all rooms, which will leave your final multipler around 3.5x or 4x, which gives you between 35k-40k just from the boss room. All this means you can average 90-100k points for full run at 500.
Just rushing to 500 using shortest path, assuming 4 rooms where the 2 middle rooms average 1.25x:
First room (1.1) + 2nd room (1.25) + 3rd room (1.25) + Boss (1.25)
12740 x [(2.1 x 5) + (2.35 x 5) + (2.6 x 5) + (2.85 x 20)] = 1,175,265
(Where 12740 = Sum of all numbers between 2-50 x 10)
Which comes down to 4 x 49 = 196 fights. Then at 500, you get 90-100k every floor clear.
Conversely, if we assume a full floor clear and reduce the average multiplier to 1.2x:
12740 x [(2.1 x 5) + (2.3 x 5) + (2.5 x 5) + (2.7 x 5) + (2.9 x 5) + (3.1 x 5) + (3.3 x 5) + (3.5 x 5) + (3.75 x 20)] = 2,382,380
Over 9 x 49 = 441 battles.
That means that for over twice the number of battles (~250 more), you would end up with 1.2-1.5mil extra, which you can get from 135-150 battles at 500. Also, the shortest paths usually involve a legendary battle, which are usually 1.3 or 1.5 multipliers so the difference between shortest path and full clears is probably lower, maybe 1-1.2 mil.