class Player extends React.Component {

    constructor(props) {
        super(props);
        this.state = {
            level: 1
        }
    }

    render() {
        return <div>Level {this.state.level}</div>
    }

    modifyLevel(delta) {
        let newLevel = this.state.level + delta;
        setState({level: newLevel});
    }

}

class App extends React.Component {

    constructor(props) {
        super(props);
        this.player = <Player/>;
    }

    render() {
        return <div class="root">
            {this.player}
        </div>
    }

    someComplexLogicMethod() {

        this.player.modifyLevel(3); // <--- here is an issue

    }

}

const element = <App/>;
ReactDOM.render(element, document.getElementById('application'));