A Sangheili Field Marshal is one of the highest Sangheili ranks within the Covenant army and is a member of the Zealot-class.
Credit to Halo Wiki
Powers and Stats
Tier: 9-A with physical blows; 9-C to 9-B with most weapons; 9-A to 8-B with heavy weapons
Name: Field Marshal
Classification: Sangheili Field Marshal
Powers and Abilities: Superhuman strength, speed, durability, agility, stamina and pain tolerance, extremely skilled and experienced combatant, master of armed and unarmed CQC, can quickly master the use of virtually all manner of weaponry, excellent military tactician
Attack Potency: Small Building level with physical blows (Comparable to but inferior to The Arbiter (Thel 'Vadam)); Street level to Wall level with most weapons; Small Building level to City Block level with various heavy weapons and demolition explosives.
Speed: Superhuman travel speed (Comparable to SPARTANs like Noble 6); likely Supersonic+ combat speed reactions (Can contend with SPARTANs in close combat, inferior to the Arbiter).
Lifting Strength: Superhuman
Striking Strength: Likely Small Building Class (Sword master who is only slightly inferior to Spartans).
Durability: Small Building level with armor; at least Small Building level+ with energy shields, possibly up to Building level (Comparable to SPARTANs).
Stamina: Superhuman (Comparable to Spartans and superior to any normal, unaugmented humans)
Range: Extended melee range, up to several hundred meters with ranged weapons
Standard Equipment: Fuel Rod Gun, Energy Sword, Type-1 Antipersonnel Plasma grenades, Field Marshal Armor
Intelligence: Excellent military tactician, skilled with many Covenant weapons
Weaknesses: Seems to be slightly cowardly, relies on his Zealot squad for full potential