EX-moves are similar to special moves only they're slightly enhanced versions. Normally they cost a portion of the special meter, however by tweaking some settings that will change. Go in to settings from the help and options section, then go to he dipswitches. Move on over to page 3/5 (Press R2 twice) and you'll see a list of things you can change. Adjust the Ex move energy requirement to 1 (which oddly enough makes it 0) and you can fire an infinite number of EX-special moves while playing in arcade or local versus matches.
Take your pick from the two and pick Ryu or Ken, then simply do a quarter circle forward paired with L1 and your character will flash and it will do two hits. If you're playing as Ryu, it will be a Red Hadouken. Simply fire these 75 times and you'll have your trophy.
If you've done some in arcade mode and go to grind these in local versus, the counters WILL be different. Your best bet is to simply knock this out in versus from the start, or continue in arcade if you've already used a decent amount of them.