A fusions weakest point is charging and it's got a very loud audio sound which while I can't really replicate the sound it starts kinda weak than gets a bit louder than the blast happens the best counter is to either A. wait till the sound stops or B. Go when you hear the loud shot sound since they have a pretty long delay long enough to allow primaries a chance to take them down. Another way is range combat since the two nerfs they got were a range and charge time nerf so if you play range it'll be tough for em not impossible however.