Exploited sexually on promise of marriage. What all can i do?

You can register a written Complaint in the Nearest Police Station against the army officer for the offences under Indian Penal Code:

1. Offence of Rape under Section 376
2. Offence of Causing miscarriage without Women's consent under Section 313
3. Offence of Criminal conspiracy under Section 120B and Attempt to murder under Section 307 (for planning to kill you)
4. Offence of assault or criminal force against women to outrage her modesty under Section 354

As per the Indian Army Act,

Civil Offence has been defined to mean any offence triable by the Criminal Court of India.

Now with respect to all such offences, 

Section 69 of the Army Act states that:

Section 69: Civil offences. Subject to the provisions of section 70, any person subject to this Act who at any place in or beyond India commits any civil offence shall be deemed to be guilty of an offence against this Act and, if charged therewith under this section, shall be liable to be tried by a court- martial and, on conviction, be punishable as follows, that is to say,-

(a) if the offence is one which would be punishable under any law in force in India with death or with transportation, he shall be liable to suffer any punishment, other than whipping, assigned for the offence, by the aforesaid law and such less punishment as is in this Act mentioned; and

(b) in any other case, he shall be liable to suffer any punishment, other than whipping, assigned for the offence by the law in force in India, or imprisonment for a term which may extend to seven years, or such less punishment as is in this Act mentioned.

Section 70 of the Army Act states that 

Section 70: Civil offence not triable by court-martial. A person subject to this Act who commits an offence of murder against a person not subject to military, naval or air force law, or of culpable homicide not amounting to murder against such a person or of rape in relation to such a person, shall not be deemed to be guilty of an offence against this Act and shall not be tried by a court- martial, unless he commits any of the said offences-

(a) while on active service, or
(b) at any place outside India, or
(c) at a frontier post specified by the Central Government by notification in this behalf.

However, Section 125 and 126 stipulate the cases where both criminal court and Court-martial have the power to exercise jurisdiction over the offence. (Your matter falls under this as the offence of rape is triable by Criminal Court while the other offence by Court-Martial).
Section 125: Choice between criminal court and court- martial. When a criminal court and a court- martial have each jurisdiction in respect of an offence, it shall be in the discretion of the officer commanding the army, army corps, division or independent brigade in which the accused person is serving or such other officer as may be prescribed to decide before which court the proceedings shall be instituted, and, if that officer decides that they should be instituted before a court- martial, to direct that the accused person shall be detained in military custody.

Section 126: Power of criminal court to require delivery of offender.

(1) When a criminal court having jurisdiction is of opinion that proceedings shall be instituted before itself in respect of any alleged offence, it may, by written notice, require the officer referred to in section 125 at his option, either to deliver over the offender to the nearest magistrate to be proceeded against according to law, or to postpone proceedings pending a reference to the Central Government.

(2) In every such case the said officer shall either deliver over the offender in compliance with the requisition, or shall forthwith refer the question as to the court before which the proceedings are to be instituted for the determination of the Central Government, whose order upon such reference shall be final.

Therefore the criminal complaint registered in the nearest Police Station shall be maintainable. And you can also send a copy of this Complaint to the Army Head Quarters as well for necessary action.