The issue is that Avalanches are slow, and the shield regen is very fast - they would take down the shield, but only if all of them hit simultaneously, which is pretty difficult to do even at the lowest dogfight speed. Medusa missiles aren't much better because of the low reload speed compared to the tesla cannon. I'd suggest either trying again with 2-3 whalers with avalanches and a Sabre lead, or have a couple dedicated tesla cannon sabres to take down the shield, then a set of follow-up craft for a second interception to crash the towers.