The game's netcode/servers quite obviously cannot handle PVP combat of Frontline's scale. Even with an extremely stable connection and low ping, you will frequently find issues such as being CC'd under purify, getting hit while covered by a paladin, and heals taking so long to go off that the target has already died by the time they trigger.

Why not adjust it to a smaller scale? A few possible examples:

8v8v8: Simple enough, 3 teams of 8.
12v12v12: Each team is made up of 3 light parties.
10v10v10: Following Crystal Conflict's example of 5 player parties, each team has 2 of these groups. Alternatively 15v15v15 with the same rationale, but that might be pushing it.