zusammenschlagen
to beat up
To violently attack and injure someone with blows, often by several people.
Die Gruppe hat den Mann grundlos zusammengeschlagen.
The group beat up the man for no reason.

to beat up / beat together
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to beat up
To violently attack and injure someone with blows, often by several people.
Die Gruppe hat den Mann grundlos zusammengeschlagen.
The group beat up the man for no reason.

'Zusammenschlagen' (to beat up / beat together) is a separable verb. In a main clause, the prefix 'zusammen' appears at the end. In subordinate clauses and in the infinitive with 'zu', the prefix and verb stay together.
Die Jungen schlugen den Schneemann aus Spaß zusammen.
The boys knocked the snowman down for fun.
The verb consists of the prefix 'zusammen' (together) and the verb 'schlagen' (to hit/beat). The prefix expresses that several things or actions come together: several attackers, two hands, two construction parts, or several ingredients.
Für den Teig schlägt man Eier und Zucker zusammen.
For the dough, you beat eggs and sugar together.
You can recognize the meaning of 'zusammenschlagen' (to beat up / beat together) from the object. If it is a person, violence is meant. With 'Händen' (hands), it is a gesture; with materials, it is crafts/workmanship; and with ingredients, it is cooking.
Er schlug die Bretter zu einer Kiste zusammen.
He hammered the boards together into a box.
The gesture of horror is almost always expressed in the fixed phrase 'die Hände über dem Kopf zusammenschlagen' (to throw one's hands up in horror). The parts 'die Hände' (the hands) and 'über dem Kopf' (over one's head) are fixed components.
Als der Chef die schlechten Zahlen sah, schlug er die Hände über dem Kopf zusammen.
When the boss saw the bad figures, he threw his hands up in horror.
to connect / to bandage
'verbinden' and 'zusammenschlagen' can both mean that parts are firmly joined together. The difference lies in the type of action. 'verbinden' is neutral and general: parts are joined physically or technically. 'zusammenschlagen' describes a rough or manual action in which you work with blows, for example with a hammer. That is why 'verbinden' fits in many situations, but 'zusammenschlagen' only fits when blows or strong hammering are really meant.

Max schlägt die verbogenen Metallteile mit einem Hammer zusammen.
Max hammers the bent metal parts together with a hammer.
Mit diesen Schrauben verbinden wir die beiden Holzplatten.
We use these screws to connect the two wooden panels.