The timing for the heavy within a combo is really tight. I have had her at 4/55 for a long time and she absolutely shreds things until the heavy in a combo fails and you pretty much get KO'd because of her squishyness. If you have Ghost, definitely rank her up since you need her for the synergy and she is great (again, until the heavy in a combo thing fails). As you play more you get better at the timing, however it still happens to me about 5% of the time despite multiple hundreds of matches with her. Perhaps screen mashers have better luck with that - I tap out each attack. Give her a try, she's definitely a great champ (until the heavy in a combo thing fails).