Assuming you've already set up your ActBlue page type (and named it something like "Donate"), your first step will be to change the slug name of your ActBlue type page (with the /entity/fundraisers link) to a different name than you're planning to use for your donation page link. This will allow you to use this slug name for your desired donation page link. 

Next, change the status of the ActBlue page from "published" to "hidden." This will make it so no one is able to navigate to the entity page through NationBuilder. 

Finally, add a new page, and make it a Redirect type page. Name it as your donation page, and insert the URL of your customized ActBlue page into the settings of your redirect type page. 

If you are using the Victory Again theme, you also likely want to change the default "Donate" button to redirect to your customized ActBlue page.  

Unless your theme has been customized, the line you need to edit to make this change will be around line 163 of your layout.html theme file. You can remove the liquid drops (the yellow code in curly brackets) and replace it with the URL for your redirect page, and what you'd like your button to be named (i.e. donate). This will look something like the following:

<div class="helper"><h4><a href="{{ site.donation_page.url }}">{{ site.donation_page.headline }}</a></h4></div>

changed to 

<div class="helper"><h4><a href="http://custom_actblue_page_url">Contribute</a></h4></div>