I did a quick search for this and didnt see it posted, if it has been i am deeply sorry.
First off, been a leecher for too long because I have not found anything worth posting, but I think this could help a little.
During the fight with rotface he spawns the slimes like normal, if you merge 3 and somehow get unlucky or are in a bad pug, you can sometimes get 2 small slimes up at once, you CAN merge both into the big slime. Just merge the first as normal, then merg any extra that you have. It helps so you dont have to dodge the explosions as much.