It takes a lot of leverage, and it looks like your multi-tool is probably pretty hard to get a good grip on. If you have a length of pipe that will slip over the end of it, try that to get a longer lever.

Honestly, most of the time I need DH's help to hold the other pedal (and when I don't, I worry that the pedals were loose ), even though I've got reasonable upper body strength and most of the experienced wrenches here will tell you I shouldn't need the help. I think some of it just depends on your frame, too, the size and geometry, how hard it is to reach through it and get a good angle on both pedals.

Then before you go to install the new pedals, put some anti-seize lubricant on the threads. If you've never done it before: use a very light coating - get a small dot of it on your finger (wearing thin gloves), smear it on the threads of the spindle, and use an old toothbrush to distribute it through the threads from inside to outside. Not so critical when it's a pedal and crankarm, obviously, but just a good habit to get into when you apply anti-seize.