Following is the timeline for short stories listed in Highschool DxD DX series relative to main series.
Highschool DxD DX Vol 1
Life. 1 (Abduction ERO!): After Vol 7
Life. 2 (Ultimate!! Onii-chan Mask): Before/After Vol 9
Life. 3 (Stop!! Yuuto-kun!): After Vol 7
Life. 4 (Records of Chichiryuutei’s Pleasure Adventure): After Vol 9
Life. 5 (Love Song to the Reincarnated Angel): After Vol 12
Life. 6 (Let’s go to Onsen): After Vol 13
Life. DX (Phoenix of the Battle School Building DX?): Between Vol 19
Highschool DxD DX Vol 2
Life. 1 (Oppai is the Sun): Straight after Vol 9
Life. 2 (Student Council’s Decision): After “Oppai is the Sun”
Life. 3 (Let’s Go with Training): After Vol 10
Life. 4 (Worship Dragon-God Girl): After Vol 13
Life. 5 (Neokmata Ninja Scroll):Within Vol 14
Life. 6 (Maniac’s Sanctuary): After Vol 17
Life. EX (Hyakki Yako and Pandemonium): After Vol 18