IMO Cobras have to be built into a strategy (like many Eldar units and formations).
- Cobras need to have other AT formations in support, and preferably to have some flexibility in how they are to be used.
- There need to be at least two and preferably three or more Cobras working together, in order to allow for enemy action and poor activation
- The list needs to be high on activations, preferably 3-4 more than opposing armies, in order to activate the Cobras safely.
- There needs to be plenty of cover for them to manoeuver behind, or preferably to 'lurk' within (being DC3, they can afford to fail a dangerous terrain test in order to gain the -1 to hit modifier). Also they will typically need to march into position in order to sustain the following turn.
- The bigger the target, the better they work. This is due to the E:A mechanics. However statistically, the number of TK hits scored by a single Cobra will always be less than the titans DC. This is why you need two or more in position.
My list had 21 activations:-
4x Rangers
4x Guardians
4x Falcons / Firestorms
4x Night-Spinners
2x Cobras
2x Vampires
1x Aspects (Ex, Aut) as the BTS
My strategy was to pin the opposition into their own half, contest one or both enemy T&H Objectives to secure a 2-0 minimum victory while retaining the activation advantage to maintain various threats. The Night Spinners and Falcons are intended to disrupt or prevent major advances, the threat of their use pinning the enemy in their own table half, while the infantry and Cobras advance behind cover.
This strategy met with mixed success during the weekend. The threat of the combined firepower did scare off some opponents, but the brittle formations did suffer against armies with significant firepower. Poor objective placement meant that the Eldar were outmanoeuvered in other games, and this left the Cobras badly out of position and ineffective.
Ultimately E:A is a game of dice and luck as demonstrated by my last game against Orks. The use of a Great Gargant and large formations meant that I had an eight activation advantage that increased during the game. At the end of turn #3 the Great Gargant was Broken, down to a single DC in strength and had three fires.
It needed a 4+ to rally (which it passed), so I then rolled the dice to see what happened to the fires. . . . .
I am renowned for throwing "1"s, but on this occasion threw 3 "6"s, which meant that The Great Gargant survived, reducing my victory from 3-0 (TSNP, DtF, BTS) to 1-0 (DtF) - which was declared a straight draw as we had no time to play another turn.