Arm cannon engaged, Rexs lines up the prism, and a coil of light lashes out, wrapping itself around the bull.

Spoiler: ooc
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Energy tether, making a touch attack to hit.

(1d20+8)[20]

Assuming a hit, it deals (2d4+1)[6] untyped damage + (1d6)[1] electricity damage, and it can't move any further away from me. It takes that damage each turn, unless it uses an action to break free (DC 15 str check.)