Full charge move but still I1.
"Warriors who charge through difficult terrain are subject to deadly salvoes of close range fire and must advance more cautiously. To represent this, if at least one model in the charging unit moved through difficult terrain as part of its charge move, all of the unit’s models must attack at Initiative step 1, regardless of other Initiative modifiers, even if the charging unit is not slowed by difficult terrain. Remember that charging models must engage as many enemies in the target unit as possible"