I found this approach to be the easiest by far Say I had a xamDatagrid bound like this, using the following test class

    public class TestClass
    {
        public Int32 Id { get; set; }
        public String Name { get; set; }
        public String Department { get; set; }
        public override string ToString()
        {
            return string.Format("{0} {1} {2}", Id, Name, Department);
        }
    }

And the grid datasource was like this

List<TestClass> classes = new List<TestClass>
  {
      new TestClass { Id = 1, Department ="xyz1", Name = "Grrr1"},
      new TestClass { Id = 2, Department ="xyz2", Name = "Grrr2"},
      new TestClass { Id = 3, Department ="xyz3", Name = "Grrr3"}
  };

  dgSearchResults.DataSource = classes;

And then I set some styles like this.

<igDP:XamDataGrid x:Name="dgSearchResults" Margin="0" Theme="RoyaleStrong"
                  Visibility="Visible" Background="{DynamicResource mainControlAreaBackGround}">




    <igDP:XamDataGrid.Resources>


        <ControlTemplate x:Key="RoundToolTip"
       TargetType="{x:Type ToolTip}">
            <Grid >
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto" />
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <Rectangle RadiusX="5"
         RadiusY="5"
         Fill="Black"
         Stroke="Black"
         StrokeThickness="1" />
                <Grid >
                    <Grid.RowDefinitions>
                        <RowDefinition Height="3" />
                        <RowDefinition Height="*" />
                        <RowDefinition Height="3" />
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition  Width="3"/>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="3"/>
                    </Grid.ColumnDefinitions>
                    <Border Grid.Column="1"
            Grid.Row="1">
                        <StackPanel>
                            <ContentControl Content="{TemplateBinding Content}" />
                        </StackPanel>
                    </Border>
                </Grid>
            </Grid>
        </ControlTemplate>

        <Style TargetType="{x:Type ToolTip}">
            <Setter Property="Template" Value="{StaticResource RoundToolTip}" />
        </Style>



        <ContentControl x:Key="ToolTipContent" Background="Transparent">
            <Border HorizontalAlignment="Stretch"
                BorderBrush="Black"
                Background="White"
                Margin="0"
                Grid.Row="0"
                Grid.Column="2"
                VerticalAlignment="Stretch"
                Height="260"
                Opacity="0.95"
                BorderThickness="5"
                CornerRadius="3" >
                <Grid>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto" />
                        <RowDefinition  Height="*"/>
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="Auto"/>
                    </Grid.ColumnDefinitions>
                    <StackPanel Background="Black" Orientation="Horizontal" HorizontalAlignment="Stretch">

                        <Label VerticalAlignment="Stretch"
                               HorizontalAlignment="Stretch"
                               Background="Black"
                               FontSize="18"
                               Foreground="White"
                               Grid.ColumnSpan="2">Payment</Label>

                        <Label VerticalAlignment="Stretch"
                               HorizontalAlignment="Stretch"
                               Background="Black"
                               FontSize="18"
                               Foreground="White"
                               Grid.ColumnSpan="2"
                               Content="{Binding Path=Cells[Id].Value}"></Label>

                        <Label VerticalAlignment="Stretch"
                               HorizontalAlignment="Stretch"
                               Background="Black"
                               FontSize="18"
                               Foreground="White"
                               Grid.ColumnSpan="2">history:</Label>

                    </StackPanel>

                    <StackPanel Grid.Row="1">
                        <StackPanel Orientation="Horizontal">
                            <Label Content="Full content"/>
                            <Label Content="{Binding}"/>   
                        </StackPanel>
                       

                    </StackPanel>
                </Grid>
            </Border>
        </ContentControl>






        <Style TargetType="{x:Type igDP:DataRecordPresenter}">
            <Setter Property="ToolTip"
                        Value="{DynamicResource ToolTipContent}" />
        </Style>

       
       
       


    </igDP:XamDataGrid.Resources>
   
    <igDP:XamDataGrid.FieldLayoutSettings>
        <igDP:FieldLayoutSettings AutoGenerateFields="True" />
    </igDP:XamDataGrid.FieldLayoutSettings>






</igDP:XamDataGrid>

Now considering how useful a tooltip is why the heck does it have to be this hard, it just shouldn't be that hard.