@stjasonl1, first thing i'd say is that as soon as you get outta there respec, heavy charge and champion are must haves for vanguard, and drop fortification like last weeks pizza. barrier is better for protection as it is affected by biotic upgrades, and geth shields are affected by tech upgrades and add 10% dmg. but better than either of those is aoe reave or tungsten ammo. shockwave/pull realtively useless, inferno ammo very useful for its aoe flinch, but wasted if you pick up ap ammo.
tactics wise, this fight is a puta because you cant charge from one platform to another, so to charge you have to run the risk of scion fire. now, if you have the claymore shotty then its worth it, drone down very fast from this attack. if you picked up AR or sniper then this fight is relegated to more of a sentinel play, stay behind cover at the very starting bit, using party member abilities to take down enemy barriers. kill order wise, it goes harbinger if hes right in front of you and you cant charge to someone else, assassins, drones, far away harbinger and leave the scion. 2nd wave, again, same thing, and keep using medigel on scion'd party members. now, it gets easier. you should have 2 scions bopping around, one on a static platform and one on a moving one. its likely that both party members are dead again at this point, do not waste medigel on them as they get pwned fast by the scions, partymembers are only needed to get rid of the drones etc. take down a scion to low health but do not kill it, then take down the 2nd to low health. the 3rd wave only spawns when a scion dies, so taking them both down low, then killing them, then reviving squaddies makes the last wave a cake walk. and with the scions, its easy to avoid their fire, just duck as soon as you hear them firing.
hope thats some help, and its how i do this fight, AR VG that loves his scimitar.