Harrow is currently my favorite Warframe for hunting the big bosses on Plains of Eidolon as he can both protect the Eidolon Lures and your team against AoE damage and also boost reload speed. I highly recommend using the Helminth to replace either the #1 or #3 ability with Roar to also get a DPS buff.
The main purpose of Harrow during the Eidolon hunt is to keep the lures alive by casting his #4 Covenant after a limb has been shot off and just before the Eidolon stand up and screams (i.e. right before his shield restores) as this will protect nearby lures and team members against ALL damage and also give you a Critical Chance bonus based on the amount of incoming damage absorbed.
As soon as you have cast the Covenent ability shift into your Operator and cloak in order to build up your damage buf from the Madurai focus schools Void Strike.
Chains enemies in front of Harrow to the ground; each chained enemy will restore Harrows shields and can provide overshields.
Augment: Tribunal – Other players will proc 50% of the effects of Penance and Thurible when attacking chained enemies.
Sacrifice shields to boost Reload Speed and Fire Rate. Damage inflicted on enemies will restore health to Harrow and nearby enemies.
Channeling ability that will restore energy to Harrow and nearby team members for each enemy killed while this ability is active. The longer it is channeled, the more energy will be restored for each killed enemy; 4x energy for headshot kills.
Augment: Warding Thurible – Nearby allies take 40% less damage while channeling Thurible and gets 0.5 extra energy when damaged.
Personally I hate channeled abilities in a fast paced game like Warframe so I have used the Helminth system to replace Thurible with Rhino’s Roar ability as this DPS buff is much more useful when fighting Eidolon bosses.
Covenent will protect Harrow and all nearby allies (incl. Eidolon Lures) against all incoming damage for a few seconds; all the absorbed damage will then be converted to a Critical Chance buf for all allies affected by Covenant.
Augment: Lasting Covenant – Headshot kills will increase the critical chance bonus duration by 3 seconds.
This build is very useful when hunting the Eidolon Teralyst / Hydralyst / Gantulyst. You must mod for a duration of about 90-95% in order to have your #4 Covenant ability expire before your team kills the next limb on the boss so you can recast it when he gets up and scream. You also want to mod for as much Strength as possible and some Efficiency. If you don’t have the Umbral mods yet you can use the normal Intensify and Vitality instead.
The Arcane Momentum gives a 60% chance on critical hits to gain 150% reload speed and the Arcane Nullifier protects you against Magnetic status effects. Both are very useful and can easily be farmed from the Eidolon bosses.