Not getting hit by the grenade in the first place would be a good start.
The chance of a Sunsinger hitting you with a Firebolt if you're in the air or moving quickly was slim to none; the only people who were severely punished by them are those who like to hardscope hallways or camp corners..The same who get killed by Shoulder Charge and are trying to get that nerfed now.
Also who said anything about Flame Shield other than you? Do you have some personal grudge against Sunsingers or something; because I never once wanted to try to defend Flame Shield.