Depends on the fight but a STARTER thunder isn't bad.

In ravana ex for example I go about
B3-t2-LL-sharp-f3-pot-f4-f4-F1-f4-f4-f3-f4-convert-f4-f4-trans/b3-sc-b4

T1/2 depending of mana ticks and time.

Squeezing in a thunder during mp down time right before or after b4 reset

Is ok from what I've seen and done. This is ravana ex specific though and I haven't had enough time practicing in Alex yet. But it doesn't seem feasible in alex atm.