If I control a Battlefield with a unit, my oponent then chooses the target with a spell (e.g. Void Seeker). If I react with a hidden here to help and summon Alpha wildclaw. Does Voidseeker go through since the target was allready chosen or can I counter/redirect the spell in that scenario?

Partly — you can't counter or redirect Void Seeker, but you can make its target untargetable, which blanks the damage entirely.

Void Seeker's target was locked in when it was finalized, so nothing re-chooses it. But Alpha Wildclaw's passive — "your units here with less Might than me can't be chosen by enemy spells and abilities" — makes the targeted unit an illegal target the moment Wildclaw lands. Per rule 758.1, a unit that becomes untargetable after being targeted causes the spell to mistarget on resolution: the damage instruction is ignored, but the caster still draws 1 (Void Seeker's "Draw 1" is a separate, unlinked instruction).

Two conditions to check: the targeted unit's Might must be less than Wildclaw's 7 (an 7+ Might unit stays legal and takes the 4), and Wildclaw must arrive at the same battlefield — a Here to Help played from facedown must play the unit to the battlefield it was hidden at (rule 811.1.d.5), which is fine since that's the battlefield holding the target.