I use 2 wrenches if the head of the bolt is slipping - I 'back up' the bolt with my finger which is usually enough to keep it from spinning as I tighten the nut, but if I feel it turn, I break out the other wrench
Yep, that's what DH did.
The process is easy;
Either kill the pump or turn the multi to 'recirc' (if you want to keep the pool running w/o filtration while you play with the filter)
Open the air relief valve and unscrew the drain plug - let filter drain (you'll never get the top off if it's full of water causing a vacuum)
Open the top dome to be sure that the sand is below the seam - you may have to scoop a little out to get it below the seam -- if it's ~ at the seam, scrape it away from the edge into the center.
While it's draining you can unscrew all the nuts and bolts (tip: reassemble the bolts and nuts with the 2 washers between them - just a couple of turns will do but it keeps those little parts from getting lost - and put them in a pail or cup for safekeeping * some of the washers may have 'bonded' to the bottom, pry them off with a screwdriver)
Remove the top of the unit.
Remove the offending gasket and wipe the seating area (top and bottom) to remove any debris and check the sealing surface for warpage or defects.
Take the new o-ring out of it's bag and lube it with a teflon or silicon based lubricant. (when lubing any o-ring BE SURE JUST TO RUB THE LUBE ONTO IT
WITHOUT PULLING NOR STRETCHING IT!!)
Seat the o-ring on the bottom 1/2 of the filter and lower the top on keeping the top as horizontal as you can (before letting the top sit on the new o-ring - sight down the bolt holes so that they line up - the less turning of the top once it's on the o-ring will prevent pinching or buckling of it)
OK here's where Hayward and I differ in our approach:
The bolts- put them in from the bottom - it makes it a lot easier to to do and only tighten them a couple of turns
Once they are all in tighten the first 4 of them in pairs N & S, E&W. Then do the same with NE & SW and NW & SE. Once you establish those 8 points the lid is on with even pressure on the o-ring and the rest can just be tightened in any order
There you go, it's done!! 8)
The last thing to do is replug the bottom drain (a little lube on that gasket is a good idea) and refill the filter with water and smooth out any sand you had to move to get the top off.
As always: if I've been less than clear in my instructions or this has generated further questions, just ask and I'll be here to answer :-D
I hope the new o-ring stops the leak - (I'm ~ sure it will

)!