Isaac Counters

Augustus turns magic damage into pure damage, which Isaac cannot absorb. For this reason he is often paired with Sebastian - but Seb himself can be countered!
Fluffy prevents his first ult, albeit Isaac will ult again pretty soon. Actually this can be exploited by using Isaac as a bait so that Seb's ult is not the one canceled. This is especially useful against teams with Augustus
Seb and Khorus can prevent his silence
Healers can support the mages until Isaac gets killed since he still suffers a sizeable portion of magic damage taken
Physical attackers can kill him: Isaac is ideally used in a tank position against mages, but he has neither the armor nor the health to tank physical damage