1.5x is extremely easy, just change the 4800 at 0292e5 to 4810 to change it from lsl to asr, which is a right shift, which divides by 2.
For 2.5x, you'll need to paste write A1 88 05 20 48 43 40 10 at 292E0. That changes the original move and load signed halfword into a single unsigned halfword load, which gives me room to load in a 5 into r0 then use the fact that the GBA's processor supports multiplication. The end result is
ldrh r1, [r4, #0x4]
mov r0, #0x5
mul r0, r1, r0
asr r0, r0, #0x1
Which multiplies the damage by 5, then divides by 2 to get 2.5x damage.
Incidentally, with this, you can set your crit multiplier to any number or any half number you care for.