Related Rule
Sierra Leone
Practice Relating to Rule 14. Proportionality in Attack
Sierra Leone’s Instructor Manual (2007) states:
III Proportionality
This seeks to balance the sometimes conflicting interests of military necessity with the requirements of humanity. It requires that a soldier, in carrying out his mission, does so without causing excessive damage to the civilian population or civilian objects.
It therefore requires an assessment of likely casualties, both military and civilian, and damage compared with the specific military advantage expected from the attack. For example, an attack on a military objective with few enemies located in a densely populated area should not be carried out if the probable damage to civilians is likely to exceed the military advantage to be gained from the attack.
… The aim is to avoid or at the very least minimize collateral damage. 
Sierra Leone, The Law of Armed Conflict. Instructor Manual for the Republic of Sierra Leone Armed Forces (RSLAF), Armed Forces Education Centre, September 2007, p. 20; see also p. 34.
[emphasis in original]
Sierra Leone’s Geneva Conventions Act (2012) states:
2. Grave breaches of the [1949 Geneva] Conventions and the [1977] First [Additional] Protocol.
(1) A person of whatever nationality commits an offence if that person, whether within or outside Sierra Leone[,] commits, aids, abets or procures any other person to commit a grave breach specified in –
(e) … paragraph … 3 … of Article 85 of the First Protocol [on, inter alia, the grave breach of launching an indiscriminate attack affecting the civilian population or civilian objects in the knowledge that such attack will cause excessive loss of life, injury to civilians or damage to civilian objects, as defined in Article 57(2)(a)(iii) of the Protocol]. 
Sierra Leone, Geneva Conventions Act, 2012, Section 2(1)(e).