Mass lings and spines. Have around 50-75% of your lings on some other place on the map to flank. Once they push up to your natural, you should have enough lings and spines to defend the front while flanking from the back. Immortals and sentries are horrible against lings, so just continue to pump lings out.
I 100% guarantee you that lings dont beat immortal sentry, even if you get a 360 degree surround forcefields will ruin your day.
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