Did not go as written, but did work out. Chrom died before doing anything to slime and Sister died first turn to Tentacle. So my third pet actually had to do a little battling to make it through. I suppose it all depends on Leonard.
This strategy can fail if you get hit with Sweep after your Chrominius dropped too low to still be your highest HP pet. (It is random when or even if the enemy casts that.) The following script quits combat if your Chrominius was swapped while the first enemy still is alive.